Memory system

ABSTRACT

According to one embodiment, a memory system includes a nonvolatile semiconductor memory, and a controller. The semiconductor memory includes a memory cell, and a write circuit configured to write data to the memory cell by applying a program voltage to the memory cell and comparing a threshold voltage of the memory cell with a first reference voltage corresponding to the write data. The write circuit is configured to execute a first programming operation to obtain a value of a write parameter by comparing the threshold voltage with a second reference voltage different from the first reference voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Applications No. 2017-183074, filed Sep. 22, 2017; andNo. 2018-033796, filed Feb. 27, 2018, the entire contents of all ofwhich are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a memory system.

BACKGROUND

In recent years, memory systems including a plurality of memory cellshave been proposed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of a memory systemaccording to the first embodiment.

FIG. 2 is a perspective view showing an example of a memory cell arrayin the memory system according to the first embodiment.

FIG. 3 is a circuit diagram showing an example of a block of the memorycell array in the memory system according to the first embodiment.

FIG. 4 is a cross-sectional view showing an example of the block of thememory cell array in the memory system according to the firstembodiment.

FIG. 5 shows an example of a threshold voltage distribution of thememory cells in the memory system according to the first embodiment.

FIG. 6 shows an example of “programming for parameter tuning” and“programming with tuned parameter” according to the first embodiment.

FIG. 7A is a flowchart showing an example of a write operation of acontroller wherein one set of write parameter values is held in aregister of a flash memory.

FIG. 7B is a flowchart showing an example of a write operation of aflash memory wherein the one set of write parameter values is held inthe register of the flash memory.

FIG. 7C is a flowchart showing an example of a write operation of thecontroller wherein two sets of write parameter values are held in theregister of the flash memory.

FIG. 7D is a flowchart showing an example of a write operation of theflash memory wherein the two sets of write parameter values are held inthe register of the flash memory.

FIG. 8 is a flowchart showing an example of a write operation of thecontroller wherein the write parameter value is held in the controller.

FIG. 9 is a block diagram showing an example of a system wherein thecontroller determines executing the “programming for parameter tuning”or the “programming with tuned parameter”.

FIG. 10 is a flowchart showing an example of the write operation of thecontroller shown in FIG. 9.

FIG. 11 shows the write operation shown in FIG. 10 in comparison with acomparative example.

FIG. 12 is a timing chart showing an example of a command sequenceindicative of the “programming for parameter tuning”.

FIG. 13 is a timing chart showing an example of a Get Feature commandsequence for getting the write parameter value from the register at the“programming for parameter tuning”.

FIG. 14 is a timing chart showing an example of a command sequenceindicative of the “programming with tuned parameter”.

FIG. 15 is a timing chart showing an example of a Set Feature commandsequence for setting the write parameter value into the register at the“programming with tuned parameter”.

FIG. 16 shows an example of a command sequence for distinguishing the“programming for parameter tuning” from the “programming with tunedparameter” by a prefix command and appending information of the writeparameter value after the prefix command.

FIG. 17 is a timing chart showing an example of a Status Read commandsequence for getting the write parameter value from the register at the“programming for parameter tuning”.

FIG. 18A and FIG. 18B show an example of a command sequence for writingdata of three pages in two steps.

FIG. 19A and FIG. 19B show an example of a threshold voltagedistribution of the memory cells after the write operation by thesequence shown in FIG. 18A and FIG. 18B.

FIG. 20A and FIG. 20B show an example of a command sequence indicativeof the “programming for parameter tuning” executed by a write methodshown in FIG. 18A and FIG. 18B.

FIG. 21A and FIG. 21B show an example of a threshold voltagedistribution after the write operation by another example of thetwo-step write command sequence.

FIG. 22A and FIG. 22B show an example of a command sequence for writingin the method shown in FIG. 21A and FIG. 21B.

FIG. 23A and FIG. 23B show an example of a command sequence indicativeof the “programming for parameter tuning” executed by the method shownin FIG. 21A and FIG. 21B.

FIG. 24 shows an example of executing the “programming for parametertuning” for the first string unit of the first word line in each group.

FIG. 25 shows an example of executing the “programming for parametertuning” for a plurality of

P/E cycles.

FIG. 26 shows an example of executing the “programming for parametertuning” for a plurality of word lines in one group with beingdistributed in a plurality of P/E cycles.

FIG. 27 shows an example of executing the “programming for parametertuning” for the first string units of a plurality of word lines in onegroup with being distributed in a plurality of P/E cycles.

FIG. 28A, FIG. 28B, FIG. 28C, FIG. 28D, and FIG. 28E show examples oftables necessary for the “programming for parameter tuning” shown inFIG. 24 to FIG. 27.

FIG. 29A and FIG. 29B show other examples of tables necessary for the“programming for parameter tuning” shown in FIG. 24 to FIG. 27.

FIG. 30 is a flowchart showing an example of a write operation atmanufacturing necessary for the “programming for parameter tuning” shownin FIG. 24 to FIG. 27.

FIG. 31 is a flowchart showing an example of the “programming forparameter tuning” shown in FIG. 24 to FIG. 27.

FIG. 32A and FIG. 32B show examples of states of a write parameter tableand an update schedule table at manufacturing, relating to the“programming for parameter tuning” shown in FIG. 24 to FIG. 27.

FIG. 33A and FIG. 33B show examples of the write parameter table and theupdate schedule table when the number of P/E cycles is 100.

FIG. 34A and FIG. 34B show examples of the write parameter table and theupdate schedule table when the number of P/E cycles is 105.

FIG. 35A and FIG. 35B show examples of the write parameter table and theupdate schedule table when the number of P/E cycles is 200.

FIG. 36A and FIG. 36B show examples of the write parameter table and theupdate schedule table when the number of P/E cycles is 205.

FIG. 37A and FIG. 37B show examples of the write parameter table and theupdate schedule table when the number of P/E cycles is 300.

FIG. 38A and FIG. 38B show examples of the write parameter table and theupdate schedule table when the number of P/E cycles is 301.

FIG. 39A and FIG. 39B show examples of the write parameter table and theupdate schedule table when the number of P/E cycles is 304.

FIG. 40A, FIG. 40B, and FIG. 40C show examples of a three-dimensionalflash memory of a two-tier structure, a group table, and an updateinterval table.

FIG. 41A, FIG. 41B, FIG. 41C, FIG. 41D, and FIG. 41E show an examplewherein elements composing a group are changed in accordance withincrease in the number of P/E cycles so that the elements of the groupare maintained to have similar write parameter values.

FIG. 42 shows an example of elements moving between groups.

FIG. 43 shows an example of group state transitions.

FIG. 44A, FIG. 44B, and FIG. 44C show examples of tables necessary forgroup recomposition.

FIG. 45A and FIG. 45B show other examples of tables necessary for grouprecomposition.

FIG. 46 is a flowchart showing an example of the write operationaccompanied by the group recomposition.

FIG. 47A is a flowchart showing an example of a first joining step inthe flowchart shown in FIG. 46.

FIG. 47B is a flowchart showing an example of a second joining step inthe flowchart shown in FIG. 46.

FIG. 48A, FIG. 48B, FIG. 48C, FIG. 48D, and FIG. 48E show an example oftable state change wherein an element not belonging to a group is joinedto a newly created group.

FIG. 49A, FIG. 49B, and FIG. 49C show an example of table state changewherein an element is not belonging to a group is joined to an existinggroup.

FIG. 50A, FIG. 50B, FIG. 50C, FIG. 50D, and FIG. 50E show an example oftable state change wherein an element belonging to a group is releasedfrom the group and joined to a newly created group.

FIG. 51A and FIG. 51B show an example of table state change wherein anelement is inspected at random and remain in the group to which theelement belongs.

FIG. 52A, FIG. 52B, FIG. 52C, and FIG. 52D show an example of tablestate change wherein an element is inspected at random, evicted from thegroup to which the element has belonged, and joined to another existinggroup.

FIG. 53A, FIG. 53B, FIG. 53C, and FIG. 53D show an example of tablestate change wherein the number of P/E cycles of elements belonging tothe group with a succession destination undefined deviates from the P/Ecycle range of the group and are released from the group.

FIG. 54A, FIG. 54B, FIG. 54C, and FIG. 54D show an example of tablestate change wherein the number of P/E cycles of elements belonging tothe group with a succession destination defined deviates from the P/Ecycle range of the group and are released from the group.

DETAILED DESCRIPTION

Embodiments will be described hereinafter with reference to theaccompanying drawings.

In general, according to one embodiment, a memory system comprises anonvolatile semiconductor memory; and a controller configured to controlthe semiconductor memory. The semiconductor memory comprises a memorycell; and a write circuit configured to write data to the memory cell byapplying a program voltage to the memory cell and comparing a thresholdvoltage of the memory cell with a first reference voltage correspondingto the write data. The write circuit is configured to execute a firstprogramming operation to obtain a value of a write parameter bycomparing the threshold voltage of the memory cell with a secondreference voltage, the second reference voltage being different from thefirst reference voltage.

The following explanations disclose examples of devices and methods toembody the technical idea of the embodiments, and the technical idea ofthe embodiments is not limited by the materials, shapes, structures,arrangements, and the like of the components explained below.Modification which is easily conceivable by a person of ordinary skillin the art comes within the scope of the disclosure as a matter ofcourse. In order to make the description clearer, the sizes, shapes andthe like of the respective parts may be changed and illustratedschematically in the drawings as compared with those in an accuraterepresentation. Constituent elements having substantially the samefunctions and features in a plurality of drawings are denoted by thesame reference numerals and their duplicate explanations may be omitted.In the following explanations, “connection” indicates not only instructconnection, but also connection made via the intervening element.

[First Embodiment]

A memory system according to the first embodiment will be explained. Amemory system including a nonvolatile semiconductor memory such as aNAND type flash memory (hereinafter called NAND flash memory) will beexplained as the memory system according to the first embodiment, butmay be a memory system including other nonvolatile memories.

[Overall Configuration]

FIG. 1 shows a schematic overall configuration of an example of a memorysystem according to the embodiment.

A memory system 1 includes, for example, a NAND flash memory chip 100serving as a nonvolatile semiconductor memory, and a controller 200. TheNAND flash memory chip 100 and the controller 200 may be constituted as,for example, a device including the same housing. Examples of the deviceare a memory card such as an SD (registered trademark) card, a solidstate drive (SSD), and the like.

The NAND flash memory chip 100 includes a plurality of nonvolatilememory cells. The controller 200 is connected to the NAND flash memorychip 100 by a NAND interface (I/F) and is connected to a host device(hereinafter called host) 300 by a host interface (I/F). The NAND I/Fincludes, for example, a chip enable signal CE, a command latch enablesignal CLE, an address latch enable signal ALE, a write enable signalWE, a read enable signal RE, a data strobe signal DQS, I/O signals DQx(DQ0 to DQ7), and ready/busy signal R/Bn. A signal may be represented byeither of a high-active (positive logic) signal and a low-active(negative logic) signal, or represented by a complementary signal of thepositive logic signal and the negative logic signal. The low-activesignal is represented by attaching “n” after the signal name forconvenience of explanations. The I/O signals DQx are used to transmitcommands, addresses, data, and the like.

The host I/F conforms to, for example, serial attached SCSI (SAS)(registered trademark). The controller 200 controls the NAND flashmemory chip 100 and accesses the NAND flash memory chip 100 in responseto a command received from the host 300. In FIG. 1, one NAND flashmemory chip 100 is connected to the controller 200 but a plurality ofNAND flash memory chips 100 may be connected to the controller 200. Thehost 300 is, for example, a digital camera, a personal computer or thelike.

[Controller 200]

The controller 200 is composed of an LSI or the like including a hostI/F controller 210, a built-in random access memory (RAM) 220, aprocessor (CPU) 230, a buffer memory 240, a NAND I/F controller 250, andan error checking and correcting (ECC) circuit 260. Each function of thecontroller 200 may be implemented by firmware executed by the processor230.

The host I/F controller 210 is connected to the host 300 via the hostI/F and transfers commands and data received from the host 300 to theprocessor 230 and the buffer memory 240. In addition, the host I/Fcontroller 210 transfers data in the buffer memory 240 to the host 300in response to a command of the processor 230.

The processor 230 controls whole operations of the controller 200. Forexample, when the processor 230 receives a write command from the host300, the processor 230 issues a write command to the NAND I/F controller250 in response to the received write command. When the processor 230receives a read command, the processor 230 issues a read command to theNAND I/F controller 250 in response to the received read command. Inaddition, the processor 230 executes various processing for managing theNAND flash memory chip 100, such as garbage collection (compaction) andwear leveling.

The NAND I/F controller 250 is connected to the NAND flash memory chip100 via the NAND I/F and controls communications with the NAND flashmemory chip 100. Then, the NAND I/F controller 250 transmits varioussignals to the NAND flash memory chip 100 and receives the signals fromthe NAND flash memory chip 100, in response to the commands receivedfrom the processor 230.

The buffer memory 240 temporarily stores the write data and the readdata. The buffer memory 240 may be composed of DRAM, SRAM, or the like.Furthermore, the buffer memory 240 may not be built in the controller200 but may be external to the controller 200.

The built-in memory 220 is, for example, a semiconductor memory such asDRAM, SRAM, or the like, and is used as a work area of the processor230. The built-in memory 220 stores firmware for managing the

NAND flash memory chip 100, various management tables, and the like.

The ECC circuit 260 executes error detection and error correction of thedata stored in the NAND flash memory chip 100. In other words, the ECCcircuit 260 generates and adds an error-correcting code to the writedata when writing the data, and decodes the error-correcting code whenreading the data.

[NAND Flash Memory Chip 100]

The NAND flash memory chip (hereinafter simply called a flash memorychip) 100 includes a memory cell array 110, a row decoder 120, a drivercircuit 130, a sense amplifier 140, an address register 150, a commandregister 160, and a sequencer 170.

The memory cell array 110 includes a plurality of physical blocks(hereinafter simply called blocks) BLK including a plurality ofnonvolatile memory cells associated with rows and columns. Four blocksBLK0 to BLK3 are shown in FIG. 1 as an example. The memory cell array110 stores data supplied from the controller 200.

The row decoder 120 selects any one of blocks BLK0 to BLK3 and selects arow in the selected block BLK.

The driver circuit 130 supplies the voltage to the selected block BLKvia the row decoder 120.

The sense amplifier 140 senses the memory cell array 110, executes anecessary operation, and outputs data DAT to the controller 200, in thedata read operation. The sense amplifier 140 transfers the write dataDAT received from the controller 200, to the memory cell array 110, inthe data write operation.

The address register 150 stores address ADD received from the controller200. The command register 160 stores command CMD received from thecontroller 200.

The sequencer 170 controls overall operations of the flash memory chip100, in response to the command CMD stored in the command register 160.More specifically, the sequencer 170 applies a predetermined voltage tothe row decoder 120, the sense amplifier 140, and the like to executewriting, reading, erasing, and the like. The sequencer 170 is connectedto a register 170A in which a write parameter value and various tablesare held. An example of the write parameter is an initial value of aprogram voltage “Initial Vpgm”. The write parameter value and varioustables may be held outside the flash memory chip 100, for example, inthe controller 200. In this case, the register 170A does not need to beprovided.

[Memory Cell Array 110]

FIG. 2 is a perspective view showing an example of the memory cell array110. For convenience of explanations, FIG. 2 shows two blocks BLK0 andBLK1 and four words lines WL0 to WL3 and does not show other blocks orword lines. For example, a data erase operation is executed for eachblock. A block is denoted by BLK when a block is generally referred to.

Each block ELK includes a source diffusion layer 42, source-side selectgate lines SGS serving as conductive layers formed above the sourcediffusion layer 42, word lines WL0 to WL3, drain-side select gate linesSGD, and an active layer AA. A plurality of NAND string units arecomposed of these layers.

The source diffusion layer 42 formed in a semiconductor substrate (notshown) is provided commonly in, for example, all of the blocks BLK inthe same plane. The source diffusion layer 42 is connected to sourceline SL-M1 via a contact plug PSL.

Three or more conductive layers formed of, for example, conductivepoly-silicon are stacked via an interlayer insulating layer (not shown)on the source diffusion layer 42. For example, six conductive layers ofthis type are stacked via the interlayer insulating layer. Eachconductive film constitutes the source-side select gate line (secondselect gate line) SGS located in the lowermost layer, the drain-sideselect gate lines (first select gate lines) SGD0 to SGD5, and the wordlines WL0 to WL3 disposed between the source-side select gate line SGSand the drain-side select gate lines SGD. In FIG. 2, the first selectgate lines SGD0 to SGD4 are hidden by other objects and not shown.

Each of five remaining conductive layers other than the drain-sideselect gate lines SGD0 to SGD5 of the uppermost layer is formed in aplate shape in one block BLK. In addition, end portions of the sixconductive layers in x direction (i.e., a direction parallel to thesurface of the semiconductor substrate) are formed in a stair shape toenable contact with each of the conductive layers. For example, the wordline WL0 is formed above the source-side select gate line SGS but is notformed above the end portion of the source-side select gate line SGS toenable contact with the source-side select gate line SGS. In otherwords, the width of the word line WL0 in the x direction is formed to benarrower than the width of the source-side select gate line SGS.Simultaneously, the width of the word line WL1 in the x direction isformed to be narrower than the width of the word line WL2, the width ofthe word line WL2 in the x direction is formed to be narrower than thewidth of the word line WL3, and the width of the word line WL3 in the xdirection is formed to be narrower than the width of the drain-sideselect gate line SGD.

Each of the drain-side select gate lines SGD0 to SGD5 of the uppermostlayer is formed in a rod shape.

The drain-side select gate lines SGD0 to SGD5 extend in the x directionand are spaced apart from each other in y direction.

The active layers (active areas) AA are formed in a columnar shape, andare formed to penetrate the source-side select gate line SGS, the wordlines WL0 to WL3, and the drain-side select gate lines SGD0 to SGD5 fromthe source diffusion layer 42 in z direction (i.e., a directionperpendicular to the surface of the semiconductor substrate). Upper endsof the active layers AA are connected to a plurality of bit lines BL0 toBL(L-1) (where L is a natural integer) that are spaced apart in the xdirection while extending in the y direction, respectively.

The source-side select gate line SGS is connected to a lead SGS-M1extending in the x direction via a contact plug PSGS. The word lines WL0to WL3 are connected to leads WL0-M1 to WL3-M1 extending in the xdirection via contact plugs PWL0 to PWL3, respectively.

The drain-side select gate lines SGD0 to SGD5 are connected to leadsSGD0-M1 to SGD5-M1 extending in the x direction via contact plugs PSGD0to PSGD5, respectively.

The bit lines BL0 to BL(L-1), and the leads SGS-M1, WL0-M1, WL1-M1 toWL3-M1, and SGD0-M1 to SGD5-M1 are formed of, for example, metallicinterconnects.

[Block BLK]

An example of a configuration of the block BLK is shown in FIG. 3. Ablock BLK includes, for example, four string units Str0 to Str3. Astring unit is generally referred to as string unit Str. Each of thestring units Str includes a plurality of NAND strings 10.

Each of the NAND strings 10 includes, for example, eight memory celltransistors MT (MT0 to MT7), and select transistors ST1 and ST2. Each ofthe memory cell transistors MT includes a control gate and a chargestorage layer, and stores data in a nonvolatile manner. The memory celltransistors MT in a NAND string 10 are serially connected between asource of the select transistor ST1 and a drain of the select transistorST2.

Gates of the select transistors ST1 in the string units Str0 to Str3 areconnected to the select gate lines SGD0 to SGD3, respectively. Incontrast, gates of the select transistors ST2 in the string units Str0to Str3 are connected commonly to, for example, the select gate lineSGS. Alternatively, the gates of the select transistors ST2 in thestring units Str0 to Str3 may be connected to the select gate line SGS0to SGS3 different for each string unit. Control gates in the memory celltransistors MT0 to MT7 in the same block BLK are connected commonly tothe word lines WL0 to WL7, respectively.

The drains of the select transistors ST1 of the NAND strings 10 in thesame column, in the memory cell array 110, are connected commonly to thebit lines BL0 to BL(L-1), respectively. In other words, the bit lines BLare connected commonly to the NAND strings 10 of the different blocksBLK. Furthermore, sources of the select transistors ST2 are connectedcommonly to the source line SL.

In other words, the string unit Str includes the NAND strings 10connected to the different bit lines BL and connected to the same selectgate line SGD. The block BLK includes the string units Str commonlyincluding the word lines WL. The memory cell array 110 includes theblocks BLK commonly including the bit lines BL. In a string unit Str,memory cell transistors MT whose gates are connected to a same word lineare called a cell unit (CU).

FIG. 4 is a cross-sectional view showing a partial area of an example ofthe block BLK. The NAND strings 10 are formed above a p-well area 20. Inother words, for example, four interconnect layers 27 functioning as theselect gate lines SGS, eight interconnect layers 23 functioning as theword lines WL0 to WL7, and, for example, four interconnect layers 25functioning as the select gate lines SGD are sequentially stacked abovethe p-well area 20. Insulating films (not shown) are formed between thestacked interconnect layers.

A plurality of pillar-shaped conductors 31 having lower ends reachingthe p-well area 20 through the interconnect layers 25, 23, and 27 areformed. To form the conductors 31, first, a plurality of memory holeshaving lower ends reaching the p-well area 20 through the interconnectlayers 25, 23, and 27 are formed by etching. In FIG. 4, the diameter ofthe memory hole is uniform in the upper layer area and the lower layerarea, and the conductor 31 is shaped in a pillar having a uniformdiameter. But, in fact, the diameter of the memory hole in the upperlayer area is larger than the diameter of the memory hole in the lowerlayer area since the upper layer area is more etched than the lowerlayer area in the multilayer film. In other words, the diameter of thememory hole gradually changes from the larger diameter to the smallerdiameter, from the upper layer area to the lower layer area. Thevariation in diameter is remarkable as the etching distance (i.e., adistance in the longitudinal direction in FIG. 4) of the memory holebecomes longer. A block insulating film 28, a charge storage layer(insulating film) 29, a gate insulating film 30, and a conductor 30 aresequentially formed inside the memory hole, and the memory celltransistor MT and the select transistors ST1 and ST2 are thereby formed.The conductor 31 functions as a current path of the NAND string 10 andis an area where channels of the respective transistors are formed. Theupper end of the conductor 31 is connected to a metallic interconnectlayer 32 which functions as the bit line BL. Thus, since the diameter ofthe memory hole in the upper layer is different from the diameter in thelower layer area, the characteristic of the memory cell transistorsdisposed in the layers different in the depth direction are different inthe flash memory of the three-dimensional structure.

An n⁺ impurity diffusion layer 33 is formed in the surface area of thep-well area 20. A contact plug 35 is formed in the diffusion layer 33.The contact plug 35 is connected to a metallic interconnect layer 36which functions as the source line SL. Furthermore, a p⁺ impuritydiffusion layer 34 is formed in the surface area of the p-well area 20.A contact plug 37 is formed on the diffusion layer 34. The contact plug37 is connected to a metallic interconnect layer 38 which functions as awell interconnect CPWELL. The well interconnect CPWELL applies anelectric potential to the conductor 31 via the well area 20.

A plurality of the above-explained structures are aligned in a direction(depth direction) perpendicular to the sheet of FIG. 4, and the stringunit Str is formed by a set of the NAND strings 10 arranged in the depthdirection.

One memory cell transistor MT may be configured to store data of 1 bitor plural bits. In a case where the memory cell transistor. MT iscapable of storing 2-bit data, 2-bit data includes a lower bit data andan upper bit data. A set of the lower bit data stored by the memory celltransistors in a cell unit is called a lower page while a set of theupper bit data stored by the memory cell transistors in a cell unit iscalled an upper page. In other words, the block BLK including eight wordlines WL has the capacity for sixteen pages. The “page” may be definedas a part of the memory space formed by the memory cell transistors in acell unit. Data write operation and data read operation are executed inunits of pages or cell units. In a case where the memory cell transistoris capable of storing 3-bit data, 3-bit data include a lower bit data, amiddle bit data, and an upper bit data. A set of the lower bit datastored by the memory cell transistors in a cell unit is called a leastsignificant bit (LSB) page or a lower page, a set of the middle bit datastored by the memory cell transistors in a cell unit is called a centersignificant bit (CSB) page or a middle page, and a set of the upper bitdata stored by the memory cell transistors in a cell unit is called amost significant (MSB) page or an upper page.

FIG. 5 is a graph showing a threshold voltage distribution which thememory cell transistors MT may have, and a voltage used in the readoperation.

As explained above, the memory cell transistors MT store 2-bit data inaccordance with the threshold voltage distribution. Data represented as2 bits are called “Er” state, “A” state, “B” state, and “C” statesequentially from the lowest voltage of the threshold voltagedistribution.

The memory cell transistors MT wherein the threshold voltagedistribution is lower than voltage VA are recognized to store the dataof “Er” state corresponding to a data erased state. The memory celltransistors MT wherein the threshold voltage distribution is higher thanthe voltage VA and lower than voltage VB (>VA) are recognized to store“A” state data. The memory cell transistors MT wherein the thresholdvoltage distribution is higher than the voltage VB and lower thanvoltage VC (>VB) are recognized to store “B” state data. The memory celltransistors MT wherein the threshold voltage distribution is higher thanor equal to the voltage VC and lower than VREAD (>VC) are recognized tostore “C” state data. The “C” state, of the 2-bit data, corresponds tothe data of the highest threshold voltage distribution.

The lower page is read by using, for example, voltages VA and VC as theread voltages. Operations of reading the data by applying the voltagesVA and VC are called read operations AR and CR, respectively. In otherwords, it is determined whether the stored data is in “Er” state or anyone of “A”, “B”, and “C” states, by the read operation AR. Furthermore,it is determined whether the stored data is in “C” state or any one of“Er”, “A”, and “B” states, by the read operation CR. In addition, theupper page is read by using the voltage VB. This is called readoperation BR. It is determined whether the stored data is in any one of“Er” and “A” states or any one of the “B” and “C” states, by the readoperation BR.

Erasing the data may be executed in the block BLK unit or in a unitsmaller than the block BLK.

Furthermore, the memory cell array 110 may have another configuration.

[Write operation in Flash Memory]

The write operation includes an operation applying a program voltage(also called a program operation) and an operation verifying thresholdvoltage (also called a program verify operation). During the programoperation, by applying the program voltage to the gate electrode of thememory cell transistor MT, electric charge is injected to the chargestorage layer of the memory cell transistor MT and the threshold voltageof the memory cell transistor is increased. During the program verifyoperation, the threshold voltage of the memory cell transistor ismonitored. Program verify operation determines whether the thresholdvoltage of the memory cell transistor has reached the predeterminedthreshold voltage or not. If the program verify fails (i.e., thethreshold voltage of the memory cell transistor does not reach thepredetermined threshold voltage), the program voltage is increased by aprogram voltage increment step AV and the program operation is executedagain. The program operation is repeated until the program verifysucceeds. The increased voltage is not limited to a constant voltage butmay be a voltage dynamically changing. Executing the write operationwhile increasing the program voltage as explained above is also calledstep-up programming.

Depending on a write parameter, such as the initial value of the programvoltage “Initial Vpgm” and the program voltage increment step, the timerequired to complete a program (i.e., time to the success in the programverify process) and/or the characteristic related to programming(hereinafter called a program characteristic) changes. For this reason,in the embodiment, a group composed of a set of the memory celltransistors is defined and, while the program operation is executed forthe memory cell transistors connected to one word line (also called acell unit) of the group or one string unit of the word line,“programming for parameter tuning” (first programming) to detect anappropriate write parameter value which achieves a desired programcharacteristic is executed. Once the appropriate write parameter valueis detected, the detected write parameter value is held, and“programming with tuned parameter” (second programming) that applies(reuses) the held write parameter value is executed for the same memorycell transistors (or other than that) in the next and subsequent writeoperations. By occasionally executing the “programming for parametertuning”, the write parameter is dynamically tuned to be an appropriatevalue. The time required for the “programming with tuned parameter” isshorter than the time required for the “programming for parametertuning”. The write parameter includes the program voltage increment stepAV, and the like other than the initial value of the program voltage“Initial Vpgm”, but in the following explanations, the initial value ofthe program voltage “Initial Vpgm” is considered as the write parameter.

[“Programming for Parameter Tuning” and “Programming With TunedParameter”]

Methods of writing to the memory cell transistors capable of storingdata of plural bits include a method of writing the data of plural bitsto the memory cell transistors in one step by one program command, and amethod of writing the data of plural bits to the memory cell transistorsin multiple steps by plural program commands. First, the method ofwriting the data of plural bits to the memory cell transistors in onestep by one program command will be explained.

FIG. 6 shows situations of the “programming for parameter tuning” andthe “programming with tuned parameter”. A pulse waveform shown on anupper side of FIG. 6 shows transition of the program voltage, and awaveform shown on a lower side of FIG. 6 shows the transition of thethreshold voltage distribution of the memory cell transistors obtainedwhen the program voltage is applied. Initial value of the programvoltage “Initial Vpgm” is applied to the word line, the program verifyis executed in order to determine whether the threshold voltagedistribution of the memory cell transistors has reached a referencevoltage. If the verify fails, the program voltage is increased by theprogram voltage increment step ΔV. If the lower portion of the thresholdvoltage distribution (i.e., the lowest threshold voltage) of the memorycell transistors exceeds the program verify reference voltage accordingto the write data, the program verify is determined to succeed and theprogram operation has ended. To cause the program verify to succeed, acondition that the number of the memory cells for program, i.e., thenumber of the memory cells wherein the threshold voltage is determinedto be lower than the program verify reference voltage is smaller than orequal to a threshold number XX should be satisfied. Here, the thresholdnumber XX may be any value.

Verify of the “A” state shown in FIG. 5 will be explained. In the“programming for parameter tuning”, smart verify using a referencevoltage different from the program verify reference voltage VA is alsoexecuted in addition to the conventional program verify. In thefollowing explanations, the smart verify reference voltage is lower thanthe program verify reference voltage VA. When the upper portion of thethreshold voltage distribution (i.e., the highest threshold voltage) ofthe memory cell transistors exceeds the smart verify reference voltage,the smart verify detection is determined to be completed. That is, thedetection is completed when the number of the memory cells beingprogrammed that have higher threshold voltages than the smart verifyreference voltage is equal to or larger than a threshold number YY. The“programming for parameter tuning” is executed until the lower portionof the threshold voltage distribution (i.e., the lowest thresholdvoltage) of the memory cell transistors exceeds the program verifyreference voltage VA. The verify operation for the other states are sameas that for the “A” state, except for the program verify referencevoltage. During the “programming for parameter tuning”, the programvoltage is detected as an optimal program voltage when the upper portionof the threshold voltage distribution of the memory cell transistorsfirst exceeds the smart verify reference voltage. By using the detectedprogram voltage as an initial value of the program voltage in the“programming with tuned parameter”, a time required for “programmingwith tuned parameter” can be reduced. During the “programming forparameter tuning”, a write parameter value which achieves a desiredperformance is detected, by gradually increasing the program voltagefrom an initial value of the program voltage “Initial Vpgm” lower thanthe preset voltage (i.e., not optimized voltage). For this reason, thenumber of loops executed until the program verify is completed duringthe “programming for parameter tuning” is more than that in the“programming with tuned parameter”. Furthermore, since the verifyoperation for the smart verify is executed in the loops during the“programming for parameter tuning”, more time is spent.

The write parameter value obtained by the “programming for parametertuning” is held in the register 170A of the flash memory chip 100 orstored in the write parameter table of the RAM 220 of the controller200. A write parameter value may be commonly applied to memory celltransistors of a group, in which the memory cells have similarcharacteristic, but may not be applied commonly to the memory celltransistors remarkably different in characteristic. For this reason, aplurality of write parameter values may exist for the overall memorycell array. An example of the write operation wherein the limited numberof write parameter settings, for example, one set of write parameters isheld in the register 170A of the flash memory chip 100 will be explainedwith reference to a flowchart of FIG. 7A and FIG. 7B. An example of thewrite operation wherein two sets of write parameters are held in theregister 170A of the flash memory chip 100 will be explained withreference to flowcharts of FIG. 7C and FIG. 7D. The register 170Aincludes a register R1 and a register R2 wherein sets of writeparameters are held respectively.

Besides the write parameter value, information indicating the group ofthe memory cell transistors which is related with the write parametervalue is also held in the register 170A. Various sets of memory celltransistors are considered as the group of the memory cell transistorsto which the same write parameter value may be applied. The group isassumed to be a block in the examples shown in FIG. 7A, FIG. 7B, FIG.7C, and FIG. 7D. The data write operation includes write operation ofthe user data received from the host 300 (hereinafter may be called“host write”) and data write operation executed by garbage collection(hereinafter may be called “GC write”). The controller 200 knows whethera current write destination block is a host write destination block or aGC write destination block.

FIG. 7A shows an example of the write operation on the controller 200side, and FIG. 7B shows the write operation on the flash memory chip 100side. As shown in step S2 of FIG. 7A, the controller 200 instructs theflash memory chip 100 to write data, designating a block address of acurrent write destination block as a write destination block address.

As shown in step S4 of FIG. 7B, when the flash memory chip 100 isinstructed to write by the controller 200, the sequencer 170 determineswhether the current write destination block address is the same as thelast write destination block address (i.e., the block address relatingto the write parameter value held in the register 170A) or not. If thedetermination result is YES (i.e., the current write destination blockaddress is the same as the last write destination block address), thesequencer 170 determines, in step S5, whether the write parameter valueis held in the register 170A or not. If the determination result is YES(i.e., the write parameter value is held in the register 170A), thesequencer 170 executes, in step S6, the “programming with tunedparameter” using the write parameter value held in the register 170A.The program voltage at the smart verify detection completion, which isthe write parameter of this block, is held in the register 170A as thewrite parameter value. By using the write parameter value as an initialprogram voltage in the “programming with tuned parameter”, the programtime can be reduced.

If the determination result in step S4 is NO (i.e., the current writedestination block address is not the same as the last write destinationblock address) or if the determination result in step S5 is NO (i.e.,the write parameter value is not held in the register 170A), thesequencer 170 executes, in step S7, the “programming for parametertuning”. The sequencer 170 stores, in step S8, the detected writeparameter value and the write destination block address into theregister 170A.

FIG. 7C shows an example of the write operation on the controller 200side wherein two sets of write parameters are held in the register 170Aof the flash memory chip 100, and FIG. 7D shows an example of the writeoperation on the flash memory chip 100 side wherein two sets of writeparameters are held in the register 170A. It is assumed that one hostwrite destination block, one GC write destination block, and anotherwrite destination block are present. Among these three write destinationblocks concurrently being written, the frequencies of executing the“programming for parameter tuning” for the host write destination blockand the GC write destination block are desired to be low, since thewrite frequencies to the host write destination block and the GC writedestination block are assumed to be higher than that to said anotherwrite destination block.

As shown in step S12 of FIG. 7C, the controller 200 determines whetherthe current write destination is the host write destination or not. Ifthe determination result is YES (i.e., the current write destinationblock is the host write destination block), the controller 200instructs, in step S14, the flash memory chip 100 to write data,designating the block address of the current write destination block(host write destination block) as the write destination block addresswithout BL prefix command attached. BL prefix command is used to selectthe register R1 or the register R2. It is assumed that the register R2is selected if BL prefix command is attached, and the register R1 isselected if BL prefix command is not attached. In the case of step S14,the register R1 is used.

If the determination result in step S12 is NO (i.e., the current writedestination block is not the host write destination block), thecontroller 200 determines, in step S16, whether the current writedestination block is the GC write destination block or not. If thedetermination result is YES (i.e., the current write destination blockis the GC write destination block), the controller 200 instructs, instep S18, the flash memory chip 100 to write data, designating the blockaddress of the current write destination block (the GC write destinationblock) as the write destination block address with BL prefix commandattached. Hence the register R2 is selected.

If the determination result in step S16 is NO (i.e., the current writedestination block is not the GC write destination block), the controller200 instructs, in step S20, the flash memory chip 100 to write data,designating the block address of the current write destination block(said another write destination block) as the write destination blockwith BL prefix command attached. Instead, the controller 200 mayinstruct, in step S20, the flash memory chip 100 to write data withoutBL prefix command attached, if the current write destination block isnot the host write destination block or the GC write destination block.Whether Blh prefix command should be attached/should not be attached maybe determined by determining which of the host write and the GC writeshould be sacrificed. For example, the write of a lower write frequencymay be sacrificed. Alternatively, a command to disable the smart verifymay be attached and the smart verify may not be executed, in step S20.

As shown in step S21 of FIG. 7D, when the flash memory chip 100 isinstructed to write data by the controller 200, the sequencer 170determines whether Blh prefix command is attached or not. If thedetermination result is NO (i.e., Blh prefix command is not attached),the sequencer 170 determines, in step S22A, whether the current writedestination block address is the same as the last write destinationblock address in the register R1 (i.e., the block address relating tothe write parameter value held in the register R1) or not. If thedetermination result is YES (i.e., the current write destination blockaddress is the same as the last write destination block address in theregister R1), the flash memory chip 100 determines, in step S23A,whether the write parameter value is held in the register R1 or not. Ifthe determination result is YES (i.e., the write parameter value is heldin the register R1), the sequencer 170 executes, in step S24A, the“programming with tuned parameter” using the write parameter value heldin the register R1. The program voltage at the smart verify detectioncompletion, which is the write parameter value of this block, is held inthe register R1 as the write parameter. By using the write parametervalue as an initial program voltage in the “programming with tunedparameter”, the program time can be reduced.

If the determination result in step S22A is NO (i.e., the current writedestination block address is not the same as the last write destinationblock address in the register R1) or if the determination result in stepS23A is NO (i.e., the write parameter value is not held in the registerR1), the sequencer 170 executes the “programming for parameter tuning”in step S26A. The sequencer 170 stores, in step S27A, the detected writeparameter value and the write destination block address into theregister R1.

If the determination result in step S21 is YES (i.e., Blh prefix commandis attached), the sequencer 170 determines, in step S22B, whether thecurrent write destination block address is the same as the last writedestination block address in the register R2 (i.e., the block addressrelating to the write parameter value held in the register R2) or not.If the determination result is YES (i.e., the current write destinationblock address is the same as the last write destination block address inthe register R2), the sequencer 170 determines, in step S23B, whetherthe write parameter value is held in the register R2 or not. If thedetermination result is YES (i.e., the write parameter value is held inthe register R2), the sequencer 170 executes, in step S24B, the“programming with tuned parameter” using the write parameter value heldin the register R2. The program voltage at the smart verify detectioncompletion, which is the write parameter of this block, is held in theregister R2 as the write parameter value. By using the write parametervalue as an initial program voltage in the “programming with tunedparameter”, the program time can be reduced.

If the determination result in step S22B is NO (i.e., the current writedestination block address is not the same as the last write destinationblock address) or if the determination result in step S23B is NO (i.e.,the write parameter value is not held in the register R2), the sequencer170 executes the “programming for parameter tuning” in step S26B. Thesequencer 170 stores, in step S27B, the detected write parameter valueand the write destination block address in the register R2.

The register holding the write parameter value is specified using Blhprefix command as shown in FIG. 7D. On the other hand, if Blh prefixcommand is not attached and in case, such as the register 170A could notbe allocated for a new write parameter set, a new write parameter valueas a detection result may overwrite an existing write parameter valueheld in the register 170A (e.g., the oldest one). For this reason, ifthe number of simultaneously used write destination blocks is largerthan the number of write parameter sets which can be held and if thewrite destination blocks are frequently changed, the write parameterheld in the register 170A is overwritten, the “programming with tunedparameter” may not be executed, and the “programming for parametertuning” needs to be executed again. Even in a case where the register170A can hold only one set of write parameter values, when writing theuser data in the block BLK0, the “programming with tuned parameter” maybe executed by executing the “programming for parameter tuning” for thefirst word line, holding the write parameter value in the register 170A,and using the write parameter value held in the register 170A for therest of word lines of the block BLK0. After that, however, when the datais written to another block, for example, the block BLK1 by, forexample, the garbage collection, the “programming for parameter tuning”for the block BLK1 is required to be executed. Furthermore, after that,the “programming for parameter tuning” is required to be executed againwhen writing the user data again to the block BLK0. An average programtime increases by execution of the “programming for parameter tuning”.

A case may be assumed where a plurality of write parameter sets is heldin the register 170A and the flash memory 100 (for example, thesequencer 170) alone controls the smart verify operations describedabove.

In this case, it may be difficult for the sequencer 170 to execute sucha complicated control as determination shown in FIG. 7C and FIG. 7D, dueto limited area and cost as a peripheral circuit of the flash memory100. For this reason, when the write destination block is changed, evenwhen two sets of write parameter values are held in the register 170A,that is, even when there exists a set of write parameter value that canbe applied to the changed write destination block, that existing set ofwrite parameter values is not utilized and the “programming forparameter tuning” may be executed again.

With reference to a flowchart of FIG. 8, an example of the writeoperation wherein the write parameter value is held in the writeparameter table of the RAM 220 of the controller 200 will be explained.The RAM 220 may store the write parameter values of all blocks as thewrite parameter tables.

In the data write operation (including user data write executed inresponse to the command received from the host 300 and data writeexecuted by the garbage collection), the controller 200 determines, instep S32, whether the write destination block has been changed from thelast write or not, based on the write destination block address. If thedetermination result is NO (i.e., the write destination block has notbeen changed), the controller 200 instructs, in step S34, the flashmemory chip 100 to execute the “programming with tuned parameter” usingthe write parameter value held in the register 170A as the initial valueof the program voltage. If the determination result in step S32 is YES(i.e., the write destination block has been changed), the controller 200determines, in step S38, whether the write parameter value of thechanged write destination block is held in the write parameter table ornot.

If the determination result in step S38 is YES (i.e., the writeparameter value of the changed write destination block is held in thewrite parameter table), the controller 200 obtains, in step S39, thewrite parameter value of the changed write destination block from thewrite parameter table and the controller 200 sets, in step S40, thewrite parameter value into the register 170A of the flash memory chip100 by using, for example, Set Feature command. After that, thecontroller 200 instructs, in step S34, the flash memory 100 to executethe “programming with tuned parameter” using the write parameter held inthe register 170A as the initial value of the program voltage.

If the determination result in step S38 is NO (i.e., the write parametervalue of the changed write destination block is not held in the writeparameter table), the controller 200 instructs, in step S42, the flashmemory chip 100 to execute the “programming for parameter tuning”. Afterthe “programming for parameter tuning” is executed and the programvoltage obtained when the upper portion of the threshold voltagedistribution of the memory cell transistors exceeds the smart verifyreference voltage held in the register 170A as the write parametervalue, the controller 200 gets, in step S43, the write parameter valuefrom the register 170A of the flash memory chip 100 by using, forexample, Get Feature command and the controller 200 stores, in step S44,the write parameter value into the write parameter table of the RAM 220.

Thus, the controller 200 executes the “programming for parameter tuning”in the first write in the block and stores the write parameter valueobtained as a result of the “programming for parameter tuning”, in thewrite parameter table, for each block. Then, the controller 200 refersto the write parameter table in the subsequent write in the same blockin the write in another block, obtains the write parameter value of theblock from the table, and executes the “programming with tunedparameter” using the obtained write parameter value as the initial valueof the program voltage, and the average program time is thereby reduced.In a case where the write parameter value is held only in the register170A of the flash memory 100, not in the write parameter table in theRAM 220 of the controller 200, the average program time becomes longerwhen the write destination block is frequently changed.

[Controller 200]

The controller 200 gets the write parameter value from the register 170Aof the flash memory chip 100 and stores the write parameter value intothe write parameter table of the RAM 220 and, oppositely, obtains thewrite parameter value from the write parameter table of the RAM 220 andstores the write parameter value into the register 170A of the flashmemory chip 100. FIG. 8 shows an example wherein the controller 200 sets(or stores) and gets (or obtains) the write parameter value by using theSet Feature command and the Get Feature command. FIG. 10 shows anexample of setting (or storing) and getting (or obtaining) the writeparameter value wherein the controller 200 executes the “programming forparameter tuning” by considering the change in the characteristic of thememory cell transistors.

The “programming for parameter tuning” is not executed when the writeparameter value is stored in the write parameter table, in FIG. 8, butFIG. 10 shows an example of executing the “programming for parametertuning” by considering the change in the characteristic of the memorycell transistors even when the write parameter value is stored in thewrite parameter table.

FIG. 9 is a block diagram showing the overall system of this example.FIG. 9 shows the details of FIG. 1 but the same elements as those shownin FIG. 1 are omitted. The memory cell 110 of the flash memory chip 100includes a plurality of (n) blocks BLK0 to BLK(n-1) (where n is anatural number). Write parameter value V0 obtained by the “programmingfor parameter tuning” for the group of the memory cell transistors isstored in the register 170A. The configuration of the rest of the flashmemory chip 100 is the same as that shown in FIG. 1.

The RAM 220 of the controller 200 stores various tables. For example,the RAM 220 stores a write parameter table 502, a group table 504, aprogram-and-erase cycle (P/E cycle) table 506, and a last “programmingfor parameter tuning” P/E cycle table 508. The RAM 220 also storesvarious data, for example, a “programming for parameter tuning”threshold value 510, hereinafter denoted by “Num”. The write parametertable 502 holds a write parameter value for each group of elements whichare expected to have similar write parameter values (e.g., the initialvalue of the program voltage) for achieving a desired programcharacteristic. An example of the group is a set of the blocks, the wordlines, or the string units. Memory cell transistors connected to eachword line may have different write parameter value (e.g., an initialvalue of the program voltage) for achieving the desired programcharacteristic and not same in a block, due to the difference indiameter of the memory hole. For this reason, an initial value may bedefined for a set of the word lines. For convenience of explanations,however, in FIG. 9, it is assumed that memory cell transistors in ablock have a same write parameter value for achieving the desiredprogram characteristic, and a group is defined as a block or a set ofblocks. Alternatively, sets of memory cell transistors of each stringunit may have a different write parameter value for achieving thedesired program characteristic from each other, and in this case, agroup may be defined as a combination of a block address (range), a wordline address (range), and a string unit address (range). The group table504 stores correspondences of group indices to block ranges to define aconstitution of a group. For example, the group table 504 storesinformation defining elements that group #0 is composed of block #0,group #1 is composed of blocks #1 and #2, and the like. If the initialvalue of the program voltage relating to the desired writecharacteristic is different for each word line of the blocks, the grouptable 504 may define the group by a combination of block(s) and wordlines. For example, the group table 504 may store information definingelements that group #0 is composed of word line #0 of block #0, group #1is composed of word line #1 of block #0, group #2 is composed of wordlines #2 to #31 of block #0, and the like. If the difference incharacteristic between blocks is small, groups may be defined for eachset of word lines and a group for a set of same word lines may be sharedamong different blocks.

Since the characteristic of the memory cell transistor may be changed asthe number of P/E cycles increases, the elements of the group expectedto have a similar initial value of the program voltage relating to thedesired program characteristic may be changed as the number of P/Ecycles increases. For example, the initial value of the program voltagerelating to the desired program characteristic is lowered as the memorycell is worn. In contrast, if an excessively high program voltage isapplied, the memory cell transistor may fall to a status calledover-program wherein the memory cell may be programmed beyond a targetthreshold voltage range. The expected data cannot be read from this cellor the data of the memory cells connected to another word line of thesame string in the same block cannot be read. For this reason, the groupdefinition is not static but may be revised in accordance with theincrease in the number of P/E cycles. Recomposing of the groups will beexplained later.

The P/E cycle table 506 stores the current number of P/E cycles for eachblock. The last “programming for parameter tuning” P/E cycle table 508stores the number of P/E cycles for each block at which the “programmingfor parameter tuning” is executed last. Since the initial value of theprogram voltage relating to the desired program characteristic of thememory cell transistors of a certain block may change in accordance withthe number of P/E cycles, executing the “programming for parametertuning” again is determined to be preferable if the difference betweenthe current number of P/E cycles and the number of P/E cycles at whichthe “programming for parameter tuning” is executed last is more than orequal to a threshold number. The threshold value 510 is the thresholdvalue Num of the number of P/E cycles for this determination.

FIG. 10 is a flowchart showing an example of the data write operationexecuted by the controller 200 configured as shown in FIG. 9. In thedata write operation (including user data write executed in response tothe command received from the host 300 and data write executed by thegarbage collection), the controller 200 identifies, in step S62, thewrite destination block from the write destination block address, andfinds the group to which the write destination block belongs byreferring to the group table 504. The controller 200 determines whetherthe write operation for the group should be executed by the “programmingfor parameter tuning” or the “programming with tuned parameter”. Acondition for executing the “programming for parameter tuning” is thatthe write parameter value of the group is not stored in the writeparameter table 502 or that P/E cycling has been executed in the numberof times more than or equal to the threshold value Num after theexecution of the last “programming for parameter tuning”. The“programming with tuned parameter” is executed in a case other thanthis.

For this reason, the controller 200 determines, in step S64, whether thewrite parameter value of the write destination group is stored in thewrite parameter table 502 or not. If the determination result is NO(i.e., the write parameter value of the write destination group is notstored in the write parameter table 502), the controller 200 instructs,in step S66, the flash memory chip 100 to execute the “programming forparameter tuning”. The flash memory chip 100 executes the “programmingfor parameter tuning” as shown in FIG. 6, and sets, into the register170A as a write parameter value, the program voltage obtained when theupper portion of the threshold voltage distribution of the memory celltransistors exceeds the smart verify reference voltage. After the“programming for parameter tuning” has finished, the controller 200gets, in step S67, the write parameter value from the register 170A ofthe flash memory chip 100 by using, for example, the Get Feature commandand the controller 200 stores, in step S68, the write parameter valueinto the write parameter table 502. In other words, the write parametervalue is output from the register 170A and input to the write parametertable 502 of the controller 200.

If the determination result in step S64 is YES (i.e., the writeparameter value of the write destination group is stored in the writeparameter table 502), in step S70, the controller 200 refers to i) thecurrent number of P/E cycles in the P/E cycle table 506, ii) the numberof P/E cycles at which the “programming for parameter tuning” isexecuted last in the last “programming for parameter tuning” P/E cycletable 508, and iii) the threshold value (Num) 510, and determineswhether P/E cycling has been executed in the number of times more thanor equal to the threshold value Num after the execution of the last“programming for parameter tuning” or not. More specifically, thecontroller 200 determines whether (the current number of P/E cycles—thenumber of P/E cycles at which the “programming for parameter tuning” isexecuted last) is more than or equal to the threshold value Num. If thedetermination result is YES (i.e., P/E cycling has been executed in thenumber of times more than or equal to the threshold value Num after theexecution of the last “programming for parameter tuning”), thecontroller 200 instructs, in step S66, the flash memory chip 100 toexecute the “programming for parameter tuning”. If the determinationresult in step S70 is NO (i.e., P/E cycling has not been executed in thenumber of times more than or equal to the threshold value after theexecution of the last “programming for parameter tuning”), it can bedetermined that the characteristic of the memory cell transistors havenot changed, and the controller 200 obtains, in step S71, the writeparameter value from the write parameter table 502 and the controller200 sets, in step S72, the write parameter value in the register 170A ofthe flash memory chip 100 by using, for example, the Set Featurecommand. In other words, the write parameter value is output from thewrite parameter table 502 of the controller 200 and input to theregister 170A. The controller 200 instructs, in step S74, the flashmemory chip 100 to execute the “programming with tuned parameter”.

In this embodiment, since the tables are stored in the RAM 220, which isvolatile, the table contents are lost upon a power interrupt of thememory system. For this reason, the “programming for parameter tuning”may be executed once for all of the groups at power-on of the memorysystem. To make the execution of the “programming for parameter tuning”at power-on of the memory system unnecessary, the tables may be storedin a nonvolatile memory. For example, the tables in the RAM 220 may bewritten to a part of the memory cell array 110 of the flash memory chip100 before power-off of the memory system. Alternatively, a nonvolatilememory may be provided in the controller 200 and the table in the RAM220 may be written to the nonvolatile memory before power-off of thememory system.

FIG. 11 shows a comparison between the write operation of a comparativeexample and the write operation executed as shown in FIG. 8. On theright side of “write sequence”, “Host write” indicates the user datawriting in response to the command received from the host 300 and “GCwrite” indicates the data writing by the garbage collection. Thecomparative example shown in the upper row of FIG. 11 shows a case wherethe write parameter table is not provided in the controller 200, andonly one write parameter set is held in the register 170A, thecontroller 200 does not control the “programming for parameter tuning”,and the flash memory chip 100 alone controls the “programming forparameter tuning”.

In the comparative example, when the user data write destination isgroup #1, the “programming for parameter tuning” is executed for thefirst word line of group #1 and obtained write parameter value V1 is setin the register 170A. The “programming with tuned parameter” is executedfor other word lines of group #1. After that, the garbage collection isexecuted and, when the write destination of the garbage collection datais group #2, the “programming for parameter tuning” is executed for thefirst word line of group #2 and obtained write parameter V2 is set inthe register 170A.

When the garbage collection has ended and the write of the user data tothe group #1 is executed again, the “programming for parameter tuning”is executed in a first write process of the group #1 in a sequence ofhost write processes after coming back from another kind of a writeprocess since the write parameter value held in the register 170A is thewrite parameter value V2 of the group #2 by the garbage collection. Theobtained write parameter value V1 is set in the register 170A. The“programming with tuned parameter” is executed for other word lines ofgroup #1. After that, the garbage collection is executed and, when thewrite destination of the garbage collection data is group #2, the“programming for parameter tuning” is executed again and the obtainedwrite parameter value V2 is set in the register 170A.

Thus, in the comparative example, the “programming for parameter tuning”is executed when the write destination group is changed.

In contrast, in the write operation shown in FIG. 8 as illustrated inthe lower row of FIG. 11, the “programming for parameter tuning” isexecuted once for the group #1 and, when the write parameter value V1 isheld in the register 170A, the controller 200 gets the write parametervalue V1 from the register 170A of the flash memory chip 100 andpreserves the write parameter value V1 in the write parameter table 502of the controller 200. After that, the garbage collection is executedand, when the write destination of the garbage collection data is group#2, the “programming for parameter tuning” is executed again and theobtained write parameter value V2 is set in the register 170A. Thecontroller 200 gets the write parameter value V2 from the register 170Aof the flash memory chip 100 and preserves the write parameter value V2in the write parameter table 502 of the controller 200.

After that, when the garbage collection has ended and the write of theuser data to the group #1 is executed again, the write parameter valueheld in the register 170A is the write parameter value V2 of the group#2 by the garbage collection. Since the write parameter value V1 of thegroup #1 is stored in the write parameter table 502 of the controller200, the controller 200 obtains the write parameter value V1 from thewrite parameter table 502 of the controller 200 and sets the writeparameter value V1 into the register 170A of the flash memory chip 100.The write operation of the user data to the group #1 is executed as the“programming with tuned parameter” reusing the write parameter value V1of the group #1 held in the register 170A as the initial value of theprogram voltage. When the user data write operation executed in responseto the command received from the host 300 has ended and the data writeoperation is executed again by the garbage collection, the writeparameter value is obtained from the write parameter table 502 of thecontroller 200 and sets the write parameter value in the register 170Aof the flash memory chip 100. Thus, the data write operation is executedas the “programming with tuned parameter”.

For this reason, in the write operation shown in FIG. 8, the frequencyof execution of the “programming for parameter tuning” is lower thanthat in the comparative example. Since the controller 200 explicitlycontrols the execution of “programming for parameter tuning” and“programming with tuned parameter” and sets (stores) and gets (obtains)the write parameter value, the frequency of the “programming forparameter tuning” can be reduced and the average program run time can bereduced by the controller 200. Furthermore, in the write operation shownin FIG. 10, the “programming for parameter tuning” is executed againwhen P/E cycling has been executed in the number of times more than orequal to the threshold value Num after execution of the last“programming for parameter tuning”. For this reason, the reliability isimproved even when the characteristic of the memory cell transistors haschanged.

COMMAND SEQUENCE EXAMPLE

An example of the command sequence used in the write operation shown inFIG. 8 will be explained. As explained above, there are various methodsfor writing data to the memory cell transistors that are capable ofstoring data of plural bits.

FIG. 12 is a timing chart showing an example of a page program commandsequence for the controller 200 instructing the flash memory chip 100 toexecute the “programming for parameter tuning” as described in step S66in FIG. 10. Each signal is represented by either of a high-active(positive logic) signal and a low-active (negative logic) signal, orrepresented by a complementary signal of the positive logic signal andthe negative logic signal. The low-active signal is represented byattaching an over line (or an upper line) with a signal indicating thesignal in FIG. 12, r the like but, in the present specification, thelow-active signal is represented by attaching “n” after the signal namefor convenience of explanations.

When the host 300 issues a write command to the controller 200, thecontroller 200 controls the NAND interface (I/F) as shown in FIG. 12 inorder to write data into a flash memory chip 100. A chip enable signalCEn is activated, a command latch enable signal CLE is activated, anaddress latch enable signal ALE is deactivated, and a read enable signalREn/RE which is the complementary signal is deactivated. A write enablesignal WEn is activated, and “8xh” is fetched in the flash memory chip100 as the “programming for parameter tuning” command via an I/O signalDQx. The I/O signal is composed of signals of plural bits, for example,DQ0 to DQ7, and the plural bits are wholly denoted as the I/O signalDQx. The command latch enable signal CLE is deactivated, the addresslatch enable signal ALE is activated, and a column address and a rowaddress of the write destination are fetched in the flash memory chip100 via the I/O signal DQx in synchronization with the activation of thewrite enable signal WEn. After that, the address latch enable signal ALEis deactivated, and write data DO, D1, . . . , DN are fetched in theflash memory chip 100 in synchronization of toaale of data strobe signalDQS/DQSn.

After the data transmission, internal chip program start command “10h”is transmitted to the flash memory chip 100 by the I/O signal DQx andthe data write is started. In write period t_(PROG), ready/busy signal(R/Bn) is set to be busy.

After that, Status Read command “70 h” is transmitted to the flashmemory chip 100 via the I/O signal DQx and it is determined whether theprogram succeeds or fails by, for example, the I/O signal DQ0. Theprogram may be determined to have been succeeded when the I/O signal DQ0is “0”, and the program may be determined to have failed when the I/Osignal DQ0 is “1”.

The number indicative of the command is a mere example and the commandis not limited to this number, similarly to the figures explained below.

FIG. 13 is a timing chart showing an example of the Get Feature commandsequence for the controller 200 getting the write parameter value fromthe register 170A of the flash memory chip 100 as described in step S67in FIG. 10.

The chip enable signal CEn is activated, and the command latch enablesignal CLE, the address latch enable signal ALE, the write enable signalWEn, and the read enable signal REn/RE are deactivated. After that, thecommand latch enable signal CLE and the write enable signal WEn areactivated, and “EEh” is transmitted via the I/O signal DQx. “EEh” of theI/O signal DQx is fetched in the flash memory chip 100 as a command insynchronization with the rise of the write enable signal WEn. Thecommand “EEh” indicates a command for starting the Get Feature sequence.The Get Feature sequence gets data R-B0, R-B1, R-B2, and R-B3 fromaddress ‘XXh” of the register 170A. The write parameter value is assumedto be held in the register 170A of the address “XXh”.

The command latch enable signal CLE is deactivated, the address latchenable signal ALE and the write enable signal WEn are activated, and“XXh” is transmitted via the I/O signal DQx. “XXh” of the I/O signal DQxis fetched in the flash memory chip 100 as an address in synchronizationwith the rise of the write enable signal WEn. After that, the addresslatch enable signal ALE is deactivated, and R-B0, R-B1, R-B2, and R-B3are transmitted to the controller 200 via the I/O signal DQx. The flashmemory chip 100 toggles the data strobe signal DQS/DQSn in response tothe controller 200 toggling the read enable signal REn/RE, and R-B0,R-B1, R-B2, and R-B3 of the I/O signal DQx are fetched as the data insynchronization of the toggle of the data strobe signal DQS/DQSn. Thefetched data is stored in the write parameter table 502. Thus, the writeparameter value is got from the register 170A by the Get Featuresequence and stored in the controller 200.

FIG. 14 is a timing chart showing an example of a page program commandsequence for the controller 200 instructing the flash memory chip 100 toexecute the “programming with tuned parameter” as described in step S74in FIG. 10. The timing chart shown in FIG. 14 is different from thetiming chart of the “programming for parameter tuning” shown in FIG. 12in that the “programming for parameter tuning” command “8xh” is changedto the “programming with tuned parameter” command “8yh”, and the samewith respect to the other features.

FIG. 15 is a timing chart showing an example of the Set Feature commandsequence for the controller 200 storing the write parameter obtainedfrom the RAM 220 of the controller 200 into the register 170A of theflash memory chip 100 as described in step S72 in FIG. 10.

The chip enable signal CEn is activated, and the command latch enablesignal CLE, the address latch enable signal ALE, the write enable signalWEn, and the read enable signal REn/RE are deactivated. After that, thecommand latch enable signal CLE and the write enable signal WEn areactivated, and “EFh” is transmitted via the I/O signal DQx. “EFh” of theI/O signal DQx is fetched in the flash memory chip 100 as a command insynchronization with the rise of the write enable signal WEn. Thecommand “EFh” indicates a command for starting the Set Feature sequence.The Set Feature sequence sets data W-B0, W-B1, W-B2, and W-B3 indicatingthe write parameter value held in the register 170A of the address “XXh”of the flash memory chip 100.

The command latch enable signal CLE is deactivated, the address latchenable signal ALE and the write enable signal WEn are activated, and“XXh” is transmitted via the I/O signal DQx. “XXh” of the I/O signal DQxis fetched in the flash memory chip 100 as an address in synchronizationwith the rise of the write enable signal WEn. After that, the addresslatch enable signal ALE is deactivated, and data W-B0, W-B1, W-B2, andW-B3 indicating the write parameter value are transmitted to the flashmemory chip 100 via the I/O signal DQx. W-B0 W-B1, W-B2, and W-B3 of theI/O signal DQx are fetched in the flash memory chip 100 as data insynchronization of the data strobe signal DQS/DQSn, and the data W-BO,W-B1, W-B2, and W-B3 are set in the address “XXh” of the register 170A.Thus, the write parameter value of the register 170A is set in theregister 170A by the Set Feature sequence.

Still other command sequences will be described. FIG. 12 and FIG. 14show the “programming for parameter tuning” sequence and the“programming with tuned parameter” sequence using the differentcommands, respectively, but a common command sequence may be employedboth for the “programming for parameter tuning” and the “programmingwith tuned parameter”, with a prefix command to indicate the“programming for parameter tuning” or the “programming with tunedparameter” as shown in FIG. 16. Further, FIG. 12 and FIG. 14 show thewrite operation of 1 bit of the plural bits written to the memory celltransistor, but the sequences shown in FIG. 12 and FIG. 14 may berepeated for the plural bits. If the plural bits are 3 bits, thesequences shown in FIG. 12 and FIG. 14 are repeated three times.Alternatively, the sequences shown in FIG. 12 and FIG. 14 may not berepeated for the plural bits, but the sequences may be distinguished byone command such that the sequence shown in FIG. 12 or FIG. 14 isexecuted once and a conventional program sequence is repeated twice.FIG. 16 shows states of the I/O signal DQx and the ready/busy signalR/Bn in 3-bit full sequence program, where all of 3 bits are programmedat once with one command. The first row, the second row, and the thirdrow correspond to the write of the data of a low-order bit (LSB page), amiddle-order bit (CSB page), and a high-order bit (MSB page),respectively. In a time-axis relationship, right end “A” in the firstrow is linked to left end “A” in the second row, and right end “B” inthe second row is linked to left end “B” in the third row.

Prefix command “YYh” show in FIG. 16 indicates, for example, the“programming for parameter tuning” and another prefix command (notshown), for example, “YZh” denotes the “programming with tunedparameter”. Thus, the “programming for parameter tuning” or the“programming with tuned parameter” may be indicated by the type of theprefix commands. Indicating the “programming for parameter tuning” orthe “programming with write after detection” may be realized by thepresence or the absence of the prefix commands, instead of usingdifferent types of prefix commands. Command “0lh” indicating the LSBpage is supplied subsequently with the prefix command “YYh” via the I/Osignal DQx and then write command “80h” is supplied. The write command“80 h” may be the same as the conventional write command. The writecommand “80 h” corresponds to “8xh” in FIGS. 12 and “8yh” in FIG. 14.Column addresses CA1 and CA2 and row addresses RA1, RA2, and RA3 aresupplied subsequently with the write command “80 h”, similarly to thesequences shown in FIG. 12 and FIG. 14. The data input subsequent withthe row address RA3 corresponds to the write data D0, D1, DN shown inFIG. 12 and FIG. 14.

Command “1Ah” after the input of the data of the LSB page is a commandfor writing the data to a register in the flash memory chip 100 (notshown). In the data write, ready/busy signal R/Bn is set to be busy.

After that, the write data of the CSB page and the MSB page aresupplied, similarly to the case of the LSB page. Command “10 h” afterthe input of the data of the MSB page is an internal chip program startcommand, and the full sequence programming is thereby started. In writeperiod t_(PROG), the ready/busy signal R/Bn is set to be busy.

FIG. 17 is a timing chart showing an example of the command sequence forthe controller 200 getting the write parameter value from the register170A of the flash memory chip 100 by using the Status Read commandinstead of the Get Feature command. The timing chart shown in FIG. 17 isdifferent from the timing chart shown in FIG. 13 in that Status Readcommand “70 h/ 78 h/F1 h/F2 h” is transmitted instead of the Get Featurecommand “EEh” and the address “XXh” is not transmitted. Furthermore, thestatus is output instead of the read data shown in FIG. 13, in thetiming chart shown in FIG. 17. An example of the status is 8-bit statusdata, and the write parameter value is indicated by using several bitsof the status data.

The command sequence is not limited to these, but a similar commandsequence may be used as the command sequence.

Next, a command sequence for writing the data of a plurality of bits tothe memory cell transistor in multiple steps by a plurality of programcommands will be explained. In the method of writing in multiple steps,the data are written to the memory cell transistor in multiple stepswhile selecting the word lines in moving forwards and backwards. Thereare two example approaches for executing the “programming for parametertuning” in the method of writing in multiple steps. The first approachis executing the “programming for parameter tuning” and the “programmingwith tuned parameter” independently for each write step. That is, foreach write step, the first approach maintains a write parameter table,executing the “programming for parameter tuning”, storing the writeparameter value into the table, and executing the “programming withtuned parameter” using the parameter value stored in the table. Thesecond approach shares a write parameter value among the write steps.That is, second approach maintains a common write parameter table whichis shared among the write steps, executing the “programming forparameter tuning” only in the first step of the write steps, storing thedetected write parameter value into the table, and executing the“programming with tuned parameter” in another write step using the writeparameter value stored in the table. In the “programming with tunedparameter” in said another write step, the write parameter value may beadjusted for said another write step, rather than using the raw valuestored in the table. The “programming with tuned parameter” may beperformed using the write parameter value stored in the table also inthe first step of the write steps.

The methods of writing in multiple steps include a method wherein thewrite pages are different in steps and a method wherein accuracies aredifferent in steps. FIG. 18A and FIG. 18B show a write command sequencefor writing the data of three pages in two steps. FIG. 18A shows thesequence of the first step of writing the data of the LSB page, and FIG.18B shows the sequence of the second step of collectively writing thedata of the CSB and MSB pages. The write time of the data write in thefirst step is denoted by t_(PROG1), and the write time of the write inthe second step is denoted by t_(PROG2). This sequence is similar to thesequence of the full sequence programming shown in FIG. 16 but isdifferent with reference to features that the sequence includes noprefix commands and that the data write of the LSB page alone is alsoexecuted. The data are written while selecting the word lines in movingforwards and backwards such that the data of the LSB page is written tothe word line #0 for the first-step, the data of the LSB page is writtento the word line #1 for the first-step, the data of the CSB page and theMSB page are written to the word line #0 for the second-step, the dataof the LSB page is written to the word line #3 for the first-step.

Also in the write of two steps, by executing the “programming with tunedparameter” using the result of the “programming for parameter tuning”the program time can be reduced. FIG. 19A shows a threshold voltagedistribution after the first write step shown in FIG. 18A. Only the dataof the LSB page is written in the first write step. The data includingthe data of the other pages are written in the second write step. FIG.19B shows a threshold voltage distribution after the second write stepshown in FIG. 18B. The first write step is different in thresholdvoltage distribution from the second write step. In the first write stepand the second write step, the smart verify reference voltage isdifferent from the program verify voltage and is, for example, aslightly lower voltage.

FIG. 20A and FIG. 20B show the command sequences of the first write step“programming for parameter tuning” and the second write step“programming for parameter tuning”. Unlike FIG. 18A and FIG. 18B, theprefix command “YYh” indicating the “programming for parameter tuning”is input before the commands “01 h” and “02 h”. The detection result isgot from the register 170A by the Get Feature sequence as shown in FIG.13 and stored in the write parameter table 502.

FIG. 21A and FIG. 21B show another example of multi-step write. In theexamples shown in FIG. 18A and FIG. 18B (or FIG. 20A and FIG. 20B),pages of writing target are different in the first write step and thesecond write step. On the other hand, in the examples shown in FIG. 21Aand FIG. 21B, accuracies of resulting threshold voltage distribution aredifferent in the first write step and the second write step. In thefirst write step, a verify voltage lower than the verify voltage definedas shown in FIG. 5 is used, the threshold voltage distributions of theseven written states are overlapped with each other as shown in FIG.21A, and a broad threshold voltage distribution can be obtained. In thesecond write step, the defined verify voltage is used and overlap of thethreshold voltage distributions is not made as shown in FIG. 21B. FIG.22A shows the first write step sequence of the two-step write as shownin FIG. 21A and FIG. 21B, and FIG. 22B shows the sequence of the secondwrite step of the two-step write as shown in FIG. 21A and FIG. 21B. Thesequences shown in FIG. 22A and FIG. 22B are similar to the sequencesshown in FIG. 16 but are different in that the prefix command “YYh”indicating the “programming for parameter tuning” is not included. Inthe example shown in FIG. 22A and FIG. 22B, the first write step and thesecond write step are common in command “80 h” indicating the program,but different commands may be used for each step, instead. The same datais input from the host 300 as write data (Data input) in the first writestep and the second write step.

FIG. 23A and FIG. 23B show the first write step sequence and the secondwrite step sequence wherein the “programming for parameter tuning” isexecuted in the two-step write shown in FIG. 22A and FIG. 22B,respectively. The sequences shown in FIG. 23A and FIG. 23B are differentfrom the sequences shown in

FIG. 22A and FIG. 22B in that the prefix command “YYh” indicating the“programming for parameter tuning” is included. Similarly to the exampleshown in FIG. 16, the “programming for parameter tuning” and the“programming with tuned parameter” are distinguished in accordance withnot only the type of the prefix command, but may be distinguished inaccordance with the presence or absence of the prefix command. In the“programming for parameter tuning” shown in FIG. 23A and FIG. 23B, thewrite time t_(PROG1) of the first write step is shorter than the writetime t_(PROG2) of the second write step. Furthermore, when the“programming for parameter tuning” and the “programming with tunedparameter” are distinguished by the prefix command, the write parametermay be transmitted to the flash memory chip 100 by adding information ofthe write parameter detected by the “programming for parameter tuning”after the prefix command indicating the “programming with tunedparameter”.

[Reducing the Frequency of “Programming for Parameter Tuning”]

Since the “programming for parameter tuning” takes more time than the“programming with tuned parameter” as explained above, the averageprogram time can be reduced by reducing the frequency of executing the“programming for parameter tuning”. However, since the characteristic ofthe memory cell transistor gradually changes as the number of P/E cyclesgrows larger, it is desired to execute the “programming for parametertuning” in an appropriate interval. For example, when the number of P/Ecycles reaches a predetermined number, a request to execute the“programming for parameter tuning” and detect the initial value of theprogram voltage on the desired write characteristic will be made. It isknown that the characteristic of the memory cell transistors connectedto adjacent word lines are similar to each other. Furthermore, it isknown that the characteristic of the memory cell transistors having thesimilar number of P/E cycles are similar to each other. Examples ofmeasures for reducing the frequency of executing the “programming forparameter tuning” (or reducing the execution count of the “programmingfor parameter tuning”) by the controller 200 instructing the“programming for parameter tuning” to be executed at appropriate timing,utilizing similarity in the characteristic of the memory celltransistors will be explained.

FIG. 24 shows a principle of a first measure example. In the firstmeasure example, the adjacent word lines considered to have similarcharacteristic in the block are grouped. A block is assumed to becomposed of 64 layers of word lines WLs. For example, word line WL0 isclassified into group Gr0, word line WL1 is classified into group Gr1,word lines WL2 to WL31 are classified into group Gr2, word lines WL32 toWL61 are classified into group Gr3, word line WL62 is classified intogroup Gr4, and word line WL63 is classified into group Gr5. This groupclassification is executed in one block, and the group classification issimilarly executed in other blocks as shown in FIG. 24. However, thenumber of groups and the number of word lines belonging to each groupmay be changed for each block. In a case where one block is composed offour string units Str0 to Str3 and the string units are written in orderfrom string unit Str0 to string unit Str3 for each word line, the“programming for parameter tuning” is executed for the memory celltransistors connected to first string unit Str0 of the first word linefirst written in each group, and the “programming with tuned parameter”is executed for the remaining memory cell transistors in the group. InFIG. 24, the numbers represent the order of write, the write of theorder represented by shading lines corresponds to the “programming forparameter tuning”, and the numbers not covered with shading linescorrespond to the “programming with tuned parameter”.

As a comparative example, an example of executing the “programming forparameter tuning” of the memory cell transistors in all of the stringunits Str0 to Str3 connected to the first word lines (WL0 in group 0,WL1 in group 1, WL2 in group 2, WL32 in group 3, WL62 in group 4, andWL63 in group 5) to which the data are first written in the respectivegroups will be considered. In the example shown in FIG. 24, the“programming for parameter tuning” count is reduced to a reciprocal ofthe number of string units connected to the word lines, and the averageprogram time is shortened as compared with the comparative example.

In the first measure example, the frequency of executing “programmingfor parameter tuning” is somewhat high since the “programming forparameter tuning” is executed in each P/E cycle. A second measureexample of further reducing the frequency of executing the “programmingfor parameter tuning” as compared to the first measure example will beexplained. The first measure example is spatially distributing theexecutions of the “programming for parameter tuning” over the word linesand/or the string units. The second measure example is temporallydistributing the executions of the “programming for parameter tuning”over the P/E cycles. The word lines are grouped in the first measureexample shown in FIG. 24 but the word lines may not be grouped in thesecond measure example shown in FIG. 25. FIG. 25 shows a principle ofthe second measure example wherein the word lines are not grouped andthe same write parameter is used for all of the string units Str0 toStr3 of each word line. In a case where the word lines are not grouped,the “programming for parameter tuning” is executed for the first stringunit Str0 of each word line and the “programming with tuned parameter”is executed for the string units Strl to Str3 other than the firststring unit Str0 of each word line.

In the second measure example, the “programming for parameter tuning” isnot executed in each P/E cycle, but the “programming for parametertuning” is executed in every several P/E cycles as shown in FIG. 25. Itis known that the characteristic of a memory cell transistor in theflash memory chip 100 changes gradually over P/E cycling, and it isexpected that the characteristic change is small for a small number ofP/E cycles. For this reason, the detection result of certain“programming for parameter tuning” may be used as the initial value ofthe program voltage of another “programming with tuned parameter” aslong as the number of P/E cycles after the detection is smaller than acertain value. In the second measure example, an average program timecan be also shortened since the execution count of the “programming forparameter tuning” is reduced.

The “programming for parameter tuning” is executed for the string unitStr0 of each word line in the second measure example shown in FIG. 25.But, if the word lines are grouped in the second measure example, the⁻programming for parameter tuning” is executed for first string unitStr0 of the first word line to which the data is first written in eachgroup. In other words, the “programming for parameter tuning” as shownin FIG. 24 is executed in N-th P/E cycle and (N+a)-th P/E cycle shown inFIG. 25.

In the second measure example shown in FIG. 25, since the “programmingfor parameter tuning” is executed for the first string unit Str0 of eachword line in the P/E cycles at every “a” times, the average program timeis long in a P/E cycle wherein the “programming for parameter tuning” isexecuted, the average program time is short in other P/E cycles, and theaverage program time changes between P/E cycles. A third measure exampleof making this change uniform will be explained with reference to FIG.26. The “programming for parameter tuning” is executed for the stringunit Str0 of all of the word lines in one P/E cycle in the secondmeasure example shown in FIG. 25. In the third measure example shown inFIG. 26, on the other hand, the number of word lines where the“programming for parameter tuning” is executed in one P/E cycle isreduced so that the ‘programming for parameter tuning” is executed forthe string unit Str0 of several word lines (one word line in theillustrated example) in each P/E cycle. For example, the “programmingfor parameter tuning” is executed for the string unit Str0 of word lineWL0 in N-th P/E cycle, (N+b)-th P/E cycle, . . . , the “programming forparameter tuning” is executed for string unit Str0 of word line WL1 in(N+1)-th P/E cycle, (N+1+b)-th P/E cycle, . . . , the “programming forparameter tuning” is executed for string unit Str0 of word line WL2 in(N+2)-th P/E cycle, (N+2+b)-th P/E cycle, Similarly, the subsequent“programming for parameter tuning” is executed for string unit Str0 ofone word line in each P/E cycle.

The third measure example shown in FIG. 26 may be modified so that the“programming for parameter tuning” is executed for all of the stringunits Str0 to Str3 of one word line in each P/E cycle.

A fourth measure example shown in FIG. 27 is a modification of the thirdmeasure example shown in FIG. 26 and indicates an example whereinadjacent word lines considered to have similar characteristic aregrouped, as in the first measure example shown in FIG. 25. In otherwords, the fourth measure example shown in FIG. 27 is an example ofdispersing executions of the “programming for parameter tuning” in oneP/E cycle in the first measure example shown in FIG. 24 into a pluralityof P/E cycles and hence the timings of the “programming for parametertuning” is dispersed also with respect to the P/E cycles. By timelydispersing the “programming for parameter tuning” similarly to the thirdand fourth measure examples, a comparatively long program operation,i.e. the “programming for parameter tuning” is not executedconsecutively at plural times in a short time of one P/E cycle, and thewrite performance (for example, the moving average of write performancewithin one second window) of the memory system 1 can be stabilized.

[Determination of Timing of Executing “Programming for ParameterTuning”]

The example of reducing the count of execution of the “programming forparameter tuning” by executing the “programming for parameter tuning”with a certain interval has been explained with reference to FIG. 26 andFIG. 27. An example wherein the controller 200 determines the timing ofthe “programming for parameter tuning” will be explained below.

FIGS. 28A to 28E and FIGS. 29A and 29B show tables necessary for thiscontrol, which are stored in the RAM 220. Also in this case, the memorycell transistors considered to have similar characteristic are groupedand the “programming for parameter tuning” is executed for each group.In a case where the characteristic of the memory cell transistors isdifferent from each other depending on the word lines connected to thetransistors, the groups are defined based on the word lines. The tablesfor certain blocks are illustrated for convenience, but the illustratedtables may be provided independently for each block. Furthermore, thememory cell transistors may be grouped more finely than the word lines,based on combination of the word lines and the string units. But thecharacteristic of memory cell transistors in different string units isassumed to be the same for convenience of explanations.

FIG. 28A shows an example of a stacked layer structure of the wordlines. Sixty-four word lines WL0 to WL63 are stacked in order from thelower side. For example, the diameter of the memory hole MH is large inthe upper layer area but may become smaller toward the lower layer area.The memory cell transistors connected to the word lines WL0 and WL1 inthe lower layer area and the memory cell transistors connected to theword lines near the central portion may be different in characteristicand are classified in different groups. Similarly, the memory celltransistors connected to the word lines WL63 and WL62 connected to theword lines near the central portion may be different in characteristicand are classified in different groups. Since the memory celltransistors connected to the word lines near the central portion aresimilar in characteristic, the memory cell transistors may be classifiedin one group or several groups.

FIG. 28B shows a group table indicating correspondences of group indicesto word line ranges included in the groups. This table corresponds tothe group table 504 shown in FIG. 9 (here, groups are defined byblocks). The group table indicates that, for example, group 0 iscomposed of word line WL0, group 1 is composed of word line WL1, group 2is composed of word lines WL2 to WL31, group 3 is composed of word linesWL32 to WL61, group 4 is composed of word line WL62, group 5 is composedof word line WL63.

FIG. 28C shows an update interval table indicating correspondences ofgroup indices to update intervals (the number of P/E cycles) set in thegroups. The update interval is an interval (or frequency) of executingthe “programming for parameter tuning”, and is indicated by the numberof P/E cycles. The update interval table indicates that, for example,the “programming for parameter tuning” for group #0 should be executedonce in 100 P/E cycles, the “programming for parameter tuning” for group#1 should be executed once in 300 P/E cycles, the “programming forparameter tuning” for group #2 should be executed once in 1000 P/Ecycles, the “programming for parameter tuning” for group #3 should beexecuted once in 1000 P/E cycles, the “programming for parameter tuning”for group #4 should be executed once in 300 P/E cycles, and the“programming for parameter tuning” for group #5 should be executed oncein 100 P/E cycles.

FIG. 28D shows a timing offset table indicating correspondences of groupindices to offset values set in the groups. The offset values are set todisperse the update timing for each group. The timing offset value tableindicates that, for example, the offset value of group #0 is 0, theoffset value of group #1 is 1, the offset value of group #2 is 2, theoffset value of group #3 is 3, the offset value of group #4 is 4, andthe offset value of group #5 is 5.

The tables shown in FIG. 28B, FIG. 28C, and FIG. 28D are not updatedonce the tables have been prepared. FIG. 28E shows a P/E cycle tableindicating correspondences of block indices to current numbers of P/Ecycles of the blocks. This table corresponds to the P/E cycle table 506shown in FIG. 9. The initial value of the number of P/E cycles is set to0 and is incremented every time the blocks are erased.

FIG. 29A shows a write parameter table indicating correspondences ofgroup indices to write parameter values indicating the initial values ofthe program voltage relating to the desired program characteristicdetected by the “programming for parameter tuning” of the groups. Thistable corresponds to the write parameter table 502 shown in FIG. 9. FIG.29B shows an update schedule table indicating correspondences of groupindices to numbers of P/E cycles at which the “programming for parametertuning” is executed last, and the numbers of P/E cycles at which a next“programming for parameter tuning” is scheduled. FIG. 29A and FIG. 29Bare updated when the “programming for parameter tuning” is executed.

FIG. 30 is a flowchart showing an example of initial setting of thewrite parameter table shown in FIG. 29A. This process is executed beforeshipment of the memory system (i.e., at the manufacturing of the memorysystem). The controller 200 selects a block in step S82 and selects aword line in step S84. The controller 200 determines, in step S86,whether the selected word line is the representative word line of thegroup defined by a set of the word lines or not.

The representative word line is a word line, to which the data patternis first written, of the word lines included in the group, for example,word line WL0. The data pattern is, for example, any random pattern.

If the determination in step S86 is YES (i.e., the selected word line isthe representative word line), the controller 200 instructs, in stepS88, the flash memory chip 100 to execute the “programming for parametertuning” for the memory cell transistors connected to the word line. Thewrite parameter obtained by the “programming for parameter tuning” isset in the register 170A of the flash memory chip 100. The controller200 gets, in step S89, the write parameter value from the register 170Aby using, for example, the Get Feature command. The controller 200stores the write parameter in the write parameter table (FIG. 29A) instep S90. The controller 200 obtains, in step S92, the update intervalfrom the update interval table (FIG. 28C), the offset value from thetiming offset value table (FIG. 28D), and the current number of P/Ecycles from the P/E cycle table (FIG. 28E). In step S94, the controller200 stores the current number of P/E cycles as the number of P/E cyclesat which the “programming for parameter tuning” is executed last, andstores (the number of P/E cycles at which the last “programming forparameter tuning” has been performed+update interval+offset value) asthe number of P/E cycles at which a next “programming for parametertuning” is scheduled in the update schedule table (FIG. 29B). Thecontroller 200 determines, in step S96, whether writing to all the wordlines of one block has ended or not.

If the determination in step S96 is NO (i.e., the writing to all theword lines has not ended), the process goes back to step S84 and thecontroller 200 selects a next word line. The process following step S86is also executed for the next word line. If the determination in stepS96 is YES (i.e., the writing to all the word lines has ended), thecontroller 200 determines, in step S98, whether writing to all theblocks has ended or not.

If the determination in step S98 is NO (i.e., the writing to all theblocks has not ended), the process goes back to step S82 and thecontroller 200 selects a next block. The processing following step S84is also executed for the next block. If the determination in step S98 isYES (i.e., the writing to all the blocks has ended), the process ends.

If the determination result in step S86 is NO (i.e., the selected wordline is not the representative word line), the controller 200 obtains,in step S101, the write parameter value from the write parameter tableand the controller 200 sets, in step S102, the write parameter valueinto the register 170A of the flash memory chip 100 by using, forexample, the Set

Feature command. The controller 200 instructs, in step S104, the flashmemory chip 100 to execute the “programming with tuned parameter” set inthe register 170A. If the “programming with tuned parameter” has ended,step S96 is executed.

FIG. 31 is a flowchart showing an example of a write operation. A useractually uses the memory system after manufacturing. When the user datawrite by command received from the host 300 or by garbage collectionstarts, the controller 200 identifies the block by the write destinationaddress and finds the group by the identified block in step S116, andthe controller 200 obtains, in step S118, the number of P/E cycles atwhich a next “programming for parameter tuning” is scheduled from theupdate schedule table

(FIG. 29B). The controller 200 identifies, in step S120, the writedestination block by the write destination address, and the controller200 finds, in step S122, the current number of P/E cycles from the P/Ecycle table (FIG. 28E).

The controller 200 determines, in step S124, whether the current numberof P/E cycles is equal to the number of P/E cycles at which a next“programming for parameter tuning” is scheduled or not. If thedetermination is YES (i.e., the current number of P/E cycles is equal tothe number of P/E cycles at which a next “programming for parametertuning” is scheduled), the current P/E cycle is a cycle wherein the“programming for parameter tuning” should be executed, and thecontroller 200 instructs, in step S126, the flash memory chip 100 toexecute the “programming for parameter tuning” for the memory celltransistors identified by the write destination address. The writeparameter value obtained by the “programming for parameter tuning” isset into the register 170A of the flash memory chip 100. The controller200 gets, in step S127, the write parameter value from the register 170Aof the flash memory chip 100 by using, for example, the Get Featurecommand and the controller 200 stores, in step S128, the acquired writeparameter value into the write parameter table (FIG. 29A).

The controller 200 obtains the update interval from the update intervaltable (FIG. 28C) in step S130. In step S132, the controller 200 storesthe current number of P/E cycles as the number of P/E cycles at whichthe “programming for parameter tuning” is executed last, and stores (thenumber of P/E cycles at which the last “programming for parametertuning” has been performed+update interval) as the number of P/E cyclesat which a next “programming for parameter tuning” is scheduled in theupdate schedule table (FIG. 29B).

If the determination in step S124 is NO (i.e., the current number of P/Ecycles is not equal to the number of P/E cycles at which a next“programming for parameter tuning” is scheduled), the current P/E cycleis a cycle wherein the “programming with tuned parameter” should beexecuted, and the controller 200 obtains, in step S135, the writeparameter value from the write parameter table and the controller 200sets, in step S136, the write parameter value into the register 170A ofthe flash memory chip 100 by using, for example, the Set Featurecommand. In step S138, the controller 200 instructs the flash memorychip 100 to execute the “programming with tuned parameter” using thewrite parameter value set in the register 170A for the memory celltransistors identified by the write destination address.

FIG. 32A to FIG. 39B show transition of the write parameter table andthe update schedule table according to increase of the number of P/Ecycles.

Before shipment of the memory system 1, the “programming for parametertuning” is executed for the representative word line of each group asdescribed in step S88 in FIG. 30. The program voltages at completion ofthe smart verify detection of groups #0 to #5 as obtained by the“programming for parameter tuning” are stored into the write parametertable as write parameter values V0_0 to V5_0, as shown in FIG. 32A. Whenthe “programming for parameter tuning” is executed before shipment ofthe memory system 1, the number of P/E cycles at this time is detectedand stored into the update schedule table, as shown in FIG. 32B. Theupdate schedule table shown in FIG. 32B is the same as the table shownin FIG. 29B.

FIG. 33A and FIG. 33B show a state of the write parameter table and astate of the update schedule table wherein the number of P/E cyclesreaches 100. The controller 200 refers to the P/E cycle table (FIG. 28E)and the update schedule table (latest state shown in FIG. 32B).According to the update schedule table, a group wherein the “programmingfor parameter tuning” is first executed is group #0 and the number ofP/E cycles at which a next “programming for parameter tuning” isscheduled is 100. For this reason, when the number of P/E cycles reaches100, the controller 200 instructs the flash memory chip 100 to executethe “programming for parameter tuning” of group #0 as described in stepS126 in FIG. 31. The initial value of the program voltage relating tothe desired program characteristic of group #0 obtained by the“programming for parameter tuning” is stored into the write parametertable as write parameter value V0_1, as shown in FIG. 33A. The currentnumber of P/E cycles (100) is stored into the update schedule table asthe number of P/E cycles at which the “programming for parameter tuning”of group #0 is executed last, as shown in FIG. 33B. Furthermore, theupdate interval (100) of group #0 is obtained from the update intervaltable shown in FIG. 28C. The obtained update interval (100) and thecurrent number of P/E cycles (100) are summed. The sum (200) is storedinto the update schedule table as the number of P/E cycles at which anext “programming for parameter tuning” of group #0 is scheduled, asshown in FIG. 33B.

FIG. 34A and FIG. 34B show a state of the write parameter table and astate of the update schedule table wherein the number of P/E cyclesreaches 105. The controller 200 refers to the P/E cycle table (FIG. 28E)and the update schedule table (latest state shown in FIG. 33B).According to the update schedule table, a group wherein the “programmingfor parameter tuning” is first executed after the number of P/E cyclesis 100 is group #5, and the number of P/E cycles at which a next“programming for parameter tuning” is scheduled is 105. For this reason,when the number of P/E cycles reaches 105, the controller 200 instructsthe flash memory chip 100 to execute the “programming for parametertuning” of group #5 as described in step S126 in FIG. 31. The initialvalue of the program voltage relating to the desired programcharacteristic of group #5 obtained by the “programming for parametertuning” is stored into the write parameter table as write parametervalue V5_1, as shown in FIG. 34A. The current number of P/E cycles (105)is stored into the update schedule table as the number of P/E cycles atwhich “programming for parameter tuning” of group #5 is executed last,as shown in FIG. 34B. Furthermore, the update interval (100) of group #5is obtained from the update interval table shown in FIG. 28C. Theobtained update interval (100) and the current number of P/E cycles(105) are summed. The sum (205) is stored into the update schedule tableas the number of P/E cycles at which next “programming for parametertuning” of group #5 is scheduled, as shown in FIG. 34B.

FIG. 35A and FIG. 35B show a state of the write parameter table and astate of the update schedule table wherein the number of P/E cyclesreaches 200. The controller 200 refers to the P/E cycle table (FIG. 28E)and the update schedule table (latest state shown in FIG. 34B).According to the update schedule table, a group wherein the “programmingfor parameter tuning” is first executed after the number of P/E cyclesis 105 is group #0, and the number of P/E cycles at which next“programming for parameter tuning” is scheduled is 200. For this reason,when the number of P/E cycles reaches 200, the controller 200 instructsthe flash memory chip 100 to execute the “programming for parametertuning” of group #0 as described in step 5126 in FIG. 31. The initialvalue of the program voltage relating to the desired programcharacteristic of group #0 obtained by the “programming for parametertuning” is stored into the write parameter table as write parametervalue V0_2, as shown in FIG. 35A. The current number of P/E cycles (200)is stored into the update schedule table as the number of P/E cycles atwhich “programming for parameter tuning” of group #0 is executed last,as shown in FIG. 35B. Furthermore, the update interval (100) of group #0is obtained from the update interval table shown in FIG. 28C. Theobtained update interval (100) and the current number of P/E cycles(200) are summed. The sum (300) is stored into the update schedule tableas the number of P/E cycles at which next “programming for parametertuning” of group #0 is scheduled, as shown in FIG. 35B.

FIG. 36A and FIG. 365 show a state of the write parameter table and astate of the update schedule table wherein the number of P/E cyclesreaches 205. The controller 200 refers to the P/E cycle table (FIG. 28E)and the update schedule table (latest state shown in FIG. 35B).According to the update schedule table, a group wherein the “programmingfor parameter tuning” is first executed after the number of P/E cyclesis 200 is group #5 and the number of P/E cycles at which next“programming for parameter tuning” is scheduled is 205. For this reason,when the number of P/E cycles reaches 205, the controller 200 instructsthe flash memory chip 100 to execute the “programming for parametertuning” of group #5 as described in step 5126 in FIG. 31. The initialvalue of the program voltage relating to the desired programcharacteristic of group #5 obtained by the “programming for parametertuning” is stored into the write parameter table as write parametervalue V5 2 as shown in FIG. 36A. The current number of P/E cycles (205)is stored into the update schedule table as the number of P/E cycles atwhich “programming for parameter tuning” of group #5 is executed last asshown in FIG. 36B. Furthermore, the update interval (100) of group #5 isobtained from the update interval table shown in FIG. 28C. The obtainedupdate interval (100) and the current number of P/E cycles (205) aresummed. The sum (305) is stored into the update schedule table as thenumber of P/E cycles at which next “programming for parameter tuning” ofgroup #5 is scheduled, as shown in FIG. 36B.

FIG. 37A and FIG. 37B show a state of the write parameter table and astate of the update schedule table wherein the number of P/E cyclesreaches 300. The controller 200 refers to the P/E cycle table (FIG. 28E)and the update schedule table (latest state shown in FIG. 36B).According to the update schedule table, a group wherein the “programmingfor parameter tuning” is first executed after the number of P/E cyclesis 205 is group #0, and the number of P/E cycles at which a next“programming for parameter tuning” is scheduled is 300. For this reason,when the number of P/E cycles reaches 300, the controller 200 instructsthe flash memory chip 100 to execute the “programming for parametertuning” of group #0 as described in step S126 in FIG. 31. The initialvalue of the program voltage relating to the desired programcharacteristic of group #0 obtained by the “programming for parametertuning” is stored into the write parameter table as write parametervalue V0_3 as shown in FIG. 37A. The current number of P/E cycles (300)is stored into the update schedule table as the number of P/E cycles atwhich the “programming for parameter tuning” of group #0 is executedlast as shown in FIG. 37B. Furthermore, the update interval (100) ofgroup #0 is obtained from the update interval table shown in FIG. 28C.The obtained update interval (100) and the current number of P/E cycles(300) are summed. The sum (400) is stored into the update schedule tableas the number of P/E cycles at a next “programming for parameter tuning”of group #0 is scheduled, as shown in FIG. 37B.

FIG. 38A and FIG. 38B show a state of the write parameter table and astate of the update schedule table wherein the number of P/E cyclesreaches 301. The controller 200 refers to the P/E cycle table (FIG. 28E)and the update schedule table (latest state shown in FIG. 37B).According to the update schedule table, a group wherein the “programmingfor parameter tuning” is first executed after the number of P/E cyclesis 300 is group #1, and the number of P/E cycles at which a next“programming for parameter tuning” is scheduled is 301. For this reason,when the number of P/E cycles reaches 301, the controller 200 instructsthe flash memory chip 100 to execute the “programming for parametertuning” of group #1 as described in step S126 in FIG. 31. The initialvalue of the program voltage relating to the desired programcharacteristic of group #1 obtained by the “programming for parametertuning” is stored into the write parameter table as write parametervalue V1_1 as shown in FIG. 38A. The current number of P/E cycles (301)is stored into the update schedule table as the number of P/E cycles atwhich the “programming for parameter tuning” of group #1 is executedlast, as shown in FIG. 38B. Furthermore, the update interval (300) ofgroup #1 is obtained from the update interval table shown in FIG. 28C.The obtained update interval (300) and the current number of P/E cycles(301) are summed. The sum (601) is stored into the update schedule tableas the number of P/E cycles at which a next “programming for parametertuning” of group #1 is scheduled, as shown in FIG. 38B.

FIG. 39A and FIG. 39B show a state of the write parameter table and astate of the update schedule table wherein the number of P/E cyclesreaches 304. The controller 200 refers to the P/E cycle table

(FIG. 28E) and the update schedule table (latest state shown in FIG.38B). According to the update schedule table, a group wherein the“programming for parameter tuning” is first executed after the number ofP/E cycles is 301 is group #4 and the number of P/E cycles at which anext “programming for parameter tuning” is scheduled is 304. For thisreason, when the number of P/E cycles reaches 304, the controller 200instructs the flash memory chip 100 to execute the ⁻programming forparameter tuning” of group #4 as described in step S126 in FIG. 31. Theinitial value of the program voltage relating to the desired programcharacteristic of group #4 obtained by the “programming for parametertuning” is stored into the write parameter table as write parametervalue V4_1 as shown in FIG. 39A. The current number of P/E cycles (304)is stored into the update schedule table as the number of P/E cycles atwhich the “programming for parameter tuning” of group #4 is executedlast, as shown in FIG. 39B. Furthermore, the update interval (300) ofgroup #4 is obtained from the update interval table shown in FIG. 28C.The update interval (300) and the current number of P/E cycles (304) aresummed. The sum (604) is stored into the update schedule table as thenumber of P/E cycles at which a next “programming for parameter tuning”of group #4 is scheduled, as shown in FIG. 39B.

After that, when the current number of P/E cycles reaches the number ofP/E cycles at which a next “programming for parameter tuning” isscheduled in the same manner as explained above, the “programming forparameter tuning” is executed, and the write parameter table and theupdate schedule table are updated.

Thus, the “programming for parameter tuning” is executed with a certaintime interval (P/E cycle interval), instead of executing in every P/Ecycle, according to the degree of the program characteristic variation,by controlling the “programming for parameter tuning” timing for eachgroup as defined as a set of word lines. For this reason, the writeparameter obtained by the “programming for parameter tuning” ismaintained to be appropriate for the memory cell transistors even if thecharacteristic changes over P/E cycles. The above explanations are basedon an assumption that the memory cell transistors have similarcharacteristic among the string units connected to the same word line.Alternatively, different groups may be defined for different stringunits in a case where the memory cell transistors have differentcharacteristic among the string units.

In the above explanations, the flash memory is assumed to have one-tierthree-dimensional structure. Recently, 2-tier flash memory as shown inFIG. 40A has been developed. The diameter of a memory hole MH becomes,for example, smaller from the upper side to the lower side in a layergroup of the upper half (tier-2), the diameter becomes, for example,smaller from the upper side to the lower side in a layer group of thelower half (tier-1), and the shape of the memory hole MH may bediscontinuous at a boundary between tier-1 and tier-2. FIG. 40B shows agroup table and FIG. 40C shows an update interval table. The group tableindicates that, for example, group #0 is composed of word lines WL0 toWL1 of tier-1, group #1 is composed of word lines WL2 to WL31 of tier-1,group #2 is composed of word lines WL32 to WL61 of tier-1, group #3 iscomposed of word lines WL62 to WL63 of tier-1, group #4 is composed ofword lines WL64 to WL65 of tier-2, group #5 is composed of word linesWL66 to WL95 of tier-2, group #6 is composed of word lines WL96 to WL125of tier-2, group #7 is composed of word lines WL126 to WL127 of tier-2,and the like. The group table of FIG. 40B corresponds to the group table504 shown in FIG. 9 (wherein the group is defined by blocks) and thegroup table shown in FIG. 28B. The update interval table indicates that,for example, the “programming for parameter tuning” for group #0 shouldbe executed once in 100 P/E cycles, the “programming for parametertuning” for group #1 should be executed once in 1000 P/E cycles, the“programming for parameter tuning” for group #2 should be executed oncein 1000 P/E cycles, the “programming for parameter tuning” for group #3should be executed once in 100 P/E cycles, the “programming forparameter tuning” for group #4 should be executed once in 100 P/Ecycles, the “programming for parameter tuning” for group #5 should beexecuted once in 1000 P/E cycles, the “programming for parameter tuning”for group #6 should be executed once in 1000 P/E cycles, and the“programming for parameter tuning” for group #7 should be executed oncein 100 P/E cycles. The update interval table of FIG. 40C corresponds tothe update interval table shown in FIG. 28C.

Allocation order of several or all addresses may be opposite to thatshown in FIG. 40A. For example, word line addresses WL0 to WL63 may beallocated from the upper to the lower layers in tier-1.

[Recomposing Groups]

As explained above, it is expected that the elements (e.g., the memorycells) in a group have similar characteristic, and a write parametervalue is stored for each group and applied to elements of the group uponwriting. To execute the “programming with tuned parameter” appropriately(i.e., being performed fast without over-program), the characteristic ofall the memory cell transistors in the group need to match or to besimilar, and the characteristic does not deviate too much from thosewherein the detection is executed. If the characteristic of all thememory cell transistors in the group are not similar, the writeparameter value detected by the “programming for parameter tuning” maynot be appropriate. In addition, when the number of P/E cycles increasesafter forming the group based on the characteristic of the memory celltransistors, the variation of the characteristic of the memory celltransistors within the group may have grown large.

An example of dynamically constituting groups sharing a write parametervalue by the controller 200 will be explained below. The controller 200forms a group with a set of blocks, word lines, or the like, wherememory cell transistors belonging therein are expected to have similarcharacteristic, and determines which of the “programming for parametertuning” and the “programming with tuned parameter” should be executedupon writing. A set of cell units such as blocks, word lines, or thelike belonging to a group is called elements. One of elements in thegroup is called a representative element, and a write parameter valuerelating to the desired program characteristic obtained by executing the“programming for parameter tuning” of the representative element iscalled a group representative value.

The controller 200 executes the “programming for parameter tuning” oncefor the representative element, in the group, detects the grouprepresentative value, and stores the group representative value. Thecontroller 200 executes the “programming with tuned parameter” using thegroup representative value for non-representative elements belonging tothe group. The group representative value is not changed afterdetermined. For this reason, suitability of the group representativevalue to the belonging elements may be lowered in accordance with thechange in characteristic of the cells. In this case, in the embodiment,the group representative value is not changed, but, instead, thebelonging group of the element is changed from the original group toanother group which has a more suitable group representative value.Thus, the controller 200 inspects whether the group representative valueis suitable for the belonging elements. If the group representativevalue is found not to be suitable for a belonging element, thecontroller 200 dynamically changes the group to which the elementbelongs. In addition, the controller 200 defines a group belongingcondition of an element for belonging to a group (hereinafter, denotedby a group condition) of the elements for each group, and regards thatthe characteristic of the memory cell transistors belonging to the groupis not changed as long as the elements satisfy the group condition.Examples of the group condition are a range of the P/E cycles (forexample, 1 to 100), a range of word lines (for example, word lines WL2to WL31), and the like. The controller 200 determines both following twoare satisfied or not; whether a write parameter value of the element(obtained by a sampling inspection), for achieving the desired programcharacteristic, matches the group representative value, and whether theelement satisfies the group condition. If the obtained write parametervalue is significantly different from the group representative value,the element is moved to a group having a group representative valuesimilar to the obtained write parameter value. After an elementbelonging to group G changes the group to which the element belongs togroup G’ as a result of the “programming for parameter tuning”, anotherelement in group G may successively change the group to which theelement belongs to group G′ without executing the “programming forparameter tuning”. This is called succession.

FIG. 41A to FIG. 41E show a summary of process of moving elementsbetween groups (i.e., changing belonging groups of the elements), inorder to maintain the suitability of the group representative value forthe elements, and to reduce the frequency of executing the “programmingfor parameter tuning”. FIG. 41A to FIG. 41E show transitions of statesof elements according to lapse of time, and FIG. 41A shows the oldeststate. Elements surrounded by double squares are targets of the“programming for parameter tuning”, and elements surrounded by singlesquares are targets of the “programming with tuned parameter”. Anexpression #P/Es denotes a number of P/E cycles of each element. In theinitial stage, as shown in FIG. 41A, since six elements #0 to #5 do notbelong to any groups, the “programming for parameter tuning” is executedfor each of elements #0 to #5 to detect an appropriate write parametervalue. Suppose that the write parameter values of elements #0 to #4 arerecognized to be similar as a result of the “programming for parametertuning”, and the controller 200 makes elements #0 to #4 belong to groupGr0 as shown in FIG. 41B. On the other hand, since the write parametervalue of element #5 is not similar to the write parameter values ofelements #0 to #4, the controller 200 makes element #5 belong to groupGr1. The elements are subjected to the sampling inspection at randomwhile the P/E cycle is repeated. Element #4 is subjected to the samplinginspection at certain timing and the write parameter value of element #4is detected as shown in FIG. 41C. If the detected write parameter valueof element #4 is more similar to the group representative value of groupGr1 than to the group representative value of group Gr0, the controller200 moves element #4 from group Gr0 to group Gr1 as shown in FIG. 41C.As a result, the elements belonging to group Gr0 become elements #0 to#3, and the elements belonging to group Gr1 become elements #4 and #5,as shown in FIG. 41D. After that, if the number of P/E cycles of acertain element, e.g. element #0 is changed from that shown in FIG. 41Aby a predetermined number (for example, 101), the “programming forparameter tuning” is executed for element #0 of group Gr0 as shown inFIG. 41E. In the state shown in FIG. 41E, since the number of P/E cyclesof each of the elements belonging to group Gr1 does not reach the updateinterval, the “programming with tuned parameter” is executed when thedata write for the elements belonging to group Gr1 occurs. The“programming for parameter tuning” based on the sampling inspection maybe executed for the elements belonging to group Gr1.

FIG. 42 shows a status of moving of an element between groups.

(a) Eviction: If an element is determined not to satisfy the groupcondition of group G as a result of sampling inspection, the controller200 evicts the element from group G (the element is removed from groupG).

(b) Releasing: If an element becomes not to satisfy the group conditionof group G as the number of P/E cycles increases, the controller 200releases the element from group G (the element is removed from group G).

(c) Succeeding: When the controller 200 releases an element from groupG, if group G′ is defined as a succession destination group of group G,the controller 200 makes the belonging group of the element succeed togroup G′ (the belonging group of the element is changed).

(d) Staying: When it is expected that the program characteristic of thememory cell transistors of an element has not changed over a certain P/Ecycles, and when it is determined that the program characteristic of thememory cell transistors of the element has not changed by a samplinginspection, the controller 200 makes the element stay in group G (i.e.,the belonging group of the element is not changed).

(e) Joining: If the controller 200 obtains a write parameter value onthe desired program characteristic of an element by executing the“programming for parameter tuning” of the element belonging to group G,the controller 200 makes the element join group G′ having the writeparameter on the desired program characteristic as the grouprepresentative value (the belonging group of the element is changed). Ifthe controller 200 releases the element from group G wherein thesuccession destination group is not defined, the controller 200registers group G′ of the joining destination group as the successiondestination group of group G.

To belong to a group, an element needs to satisfy the group condition ofthe group. The element is moved in the following manner according towhether the element satisfies the group condition or not. The elementbelongs to only one group but does not belong to a plurality of groupsat a time. In other words, the group conditions of any two groups amongall the groups do not overlap.

When the controller 200 detects that an element does not satisfy thegroup condition of the belonging group of the element as a result ofsampling inspection, the controller 200 evicts the element from thebelonging group. When the element is found not to satisfy the groupcondition of the belonging group in a situation other than the samplinginspection, the controller 200 releases the element from the belonginggroup. For example, if the group condition is the number of P/E cycleswhich increases as elapse of the time, the element may not satisfy thegroup condition of the belonging group when the number of P/E cyclesincreases. The controller 200 determines whether the element satisfiesthe group condition of the belonging group or not. If the controller 200determines that the element satisfies the group condition, thecontroller 200 makes the element stay in the belonging group. If thecontroller 200 determines that the element does not satisfy the groupcondition of the belonging group, the controller 200 determines whetherthe element satisfies the group condition of another group or not. Ifthe controller 200 determines that the element satisfies the groupcondition of said another group, the controller 200 makes the elementjoin said another group wherein the element satisfies the groupcondition (change the belonging group). When the controller 200 is tochange the belonging group of the element, if there is no existing groupwith a group condition that the element satisfies, the controller 200creates a new group for the element. When a group is defined as thesuccession destination of the belonging group of the element, thecontroller 200 releases the element from the belonging group and causesthe element to belong to the succession destination group (i.e., theelement follows a succession relation of the group). FIG. 43 shows anexample of state transitions of a group. A “not exist” state indicates agroup not generated. When a group representative value is defined forthe “not exist” group, transition from the “not exist” state to a“succession destination undefined” state is executed. When a successiondestination is defined for the “succession destination undefined” group,transition from the “succession destination undefined” state to a“succession destination defined” state is executed. When it is assumedthat any element in the memory system will not belong to a group, thegroup may be deleted. For example, it is assumed that the P/E cyclerange of group Gj is 101-200 and the number of P/E cycles of allelements in the memory system is 201 or more. The number of P/E cyclesof all elements in the memory system will not be 101-200. Therefore, thegroup Gj may be deleted.

FIG. 44A to FIG. 44C and FIG. 45A to FIG. 45B show examples of tables,stored in the RAM 220, used to control moving elements between groups.FIG. 44A shows an element-group table indicating correspondences ofelements to group indices and detection reservation flags. The elementsare specified by the block indices and the word line ranges. Thedetection reservation flag indicates whether the sampling inspectionneeds to be executed or not to determine whether the element satisfiesthe group condition or not. The sampling inspection is executed when theflag is “1”, and the sampling inspection is not executed when the flagis “0”.

The element-group table indicates that, for example, element (word lineWL0 of block #0) is included in group #0, element (word line WL1 ofblock #0) is included in group #1, element (word lines WL2 to WL31 ofblock #0) is included in group #2, element (word lines WL32 to WL61 ofblock #0) is included in group #3, element (word line WL62 of block #0)is included in group #4, element (word line WL63 of block #0) isincluded in group #5, element (word line WL0 of block #1) is included ingroup #6, element (word line WL1 of block #1) is included in group #7,element (word lines WL2 to WL31 of block #1) is included in group #8,detection reservation flags of all elements shown in FIG. 44A are 0, andthe like.

FIG. 44B shows a group table indicating correspondences of group indicesto word line ranges, P/E cycle ranges, and group representative values.The word line range is included in the group condition in order to makethe group condition more strict when the characteristic variation islarge. The element should not join only because its write parametervalue is the same as the representative value of the group at a certaintime. The word line ranges and the P/E cycle ranges are included in thegroup condition of the group. Each of the word line ranges and the P/Ecycle ranges is called a sub-condition. A conjunction of thesub-conditions corresponds to the group condition. If the word line andthe P/E cycle of an element fall within these ranges, the elementsatisfies the group condition.

The group table indicates, for example, that the word line range ofgroup #0 is word line #0, the P/E cycle range of group #0 is 0 to 100,and the group representative value of group #0 is V1, that the word linerange of group #1 is word line #1, the P/E cycle range of group #1 is 0to 300, and the group representative value of group #1 is V2, that theword line range of group #2 is word lines #2 to #31, the P/E cycle rangeof group #2 is 0 to 1000, and the group representative value of group #2is V3, that the word line range of group #3 is word lines #32 to #61,the P/E cycle range of group #3 is 0 to 1000, and the grouprepresentative value of group #3 is V4, that the word line range ofgroup #4 is word line #62, the P/E cycle range of group #4 is 0 to 300,and the group representative value of group #4 is V5, that the word linerange of group #5 is word line #63, the P/E cycle range of group #5 is 0to 100, and the group representative value of group #5 is V6, that theword line range of group #6 is word line #0, the P/E cycle range ofgroup #6 is 101 to 200, and the group representative value of group #6is V7, that the word line range of group #7 is word line #1, the P/Ecycle range of group #7 is 301 to 600, and the group representativevalue of group #7 is V4, that the word line range of group #8 is wordlines #2 to #31, the P/E cycle range of group #8 is 1001 to 2000, andthe group representative value of group #8 is V5, that the word linerange of group #9 is word lines #32 to #61, the P/E cycle range of group#9 is 1001 to 2000, and the group representative value of group #9 isV6, and the like.

The reason why a word line range is included in a group condition is asfollows. In a case where the word line range is not included in thegroup condition irrespective of large characteristic variation of theword lines, the element may belong to the group having the grouprepresentative value which accidentally matches the write parameter ofthe element at a certain time in the joining and, after that, even whenthe characteristic has changed and deviated from the original value, thechange in the characteristic of the element may not be found (until theelement becomes a target of the sampling inspection) and the“programming with tuned parameter” may be executed by using aninappropriate write parameter value.

FIG. 44C shows a succession destination table indicating correspondencesof group indices to succession destination groups. The successiondestination table indicates that, for example, the successiondestination of group #0 is not defined, the succession destination ofgroup #1 is not defined, the succession destination of group #2 is group#8, the succession destination of group #3 is not defined, and thesuccession destination of group #4 is not defined.

FIG. 45A shows a P/E cycle table indicating correspondences of blockindices to current numbers of P/E cycles. The P/E cycle table indicatesthat, for example, the number of P/E cycles of block #0 is 20, thenumber of P/E cycles of block #1 is 35, the number of P/E cycles ofblock #2 is 25, and the number of P/E cycles of block #3 is 80.

FIG. 45B shows a P/E cycle interval table indicating correspondences ofword line ranges to P/E cycle intervals to set the group condition. TheP/E cycle interval table indicates, for example, that the P/E cycleinterval for the word line #0 is 100, the P/E cycle interval for theword line #1 is 300, the P/E cycle interval for the word line ranges ofword lines #2 to #31 is 1000, the P/E cycle interval for the word lineranges of word lines #32 to #61 is 1000, the P/E cycle interval for wordline #62 is 300, the P/E cycle interval for word line #63 is 100, andthe like.

FIG. 46 shows a flowchart for explanation of a process example of grouprecomposition based on moving element between groups. When the write ofuser data by the command received from the host 300 or by the garbagecollection starts, the controller 200 determines, in step S154, whetherthe element (block and word line) corresponding to the address of thewrite destination belongs to a group or not, by referring to theelement-group table (FIG. 44A). If the determination is NO (i.e., theelement does not belong to any group), the controller 200 executes firstjoining process in step S156.

FIG. 47A is a flowchart showing an example of the first joining processin steps S156 and S170, and FIG. 47B is a flowchart showing an exampleof second joining process in step S162.

In the first joining process shown in FIG. 47A, the controller 200instructs, in step S202, the flash memory chip 100 to execute the“programming for parameter tuning” for an element. The write parametervalue obtained by the “programming for parameter tuning” is set into theregister 170A and then the controller 200 obtains the write parametervalue. In step S204, the controller 200 determines whether or not thegroup representative value matching the write parameter value obtainedby the “programming for parameter tuning” exists in the group table(FIG. 44B) (assumed to exist in a group Grk) and the element satisfiesthe group condition of the group Grk. Matching is not limited to strictequivalence. The group representative value may be considered to matchthe write parameter value, if the difference is within an allowablerange. If the determination in step S204 is YES (i.e., the grouprepresentative value exists and the element satisfies the groupcondition), the controller 200 updates, in step S206, each table to makethe element belong to the group Grk. An example of updating the tablewill be explained later.

If the determination in step S204 is NO (i.e., the group representativevalue does not exist or the element does not satisfy the groupcondition), the controller 200 updates, in step S208, each table tocreate a new group Grk including the write parameter value obtained bythe “programming for parameter tuning” as the group representativevalue. The group condition of the new group Grk is set by referring tothe P/E cycle table, the P/E cycle interval table, and the element-grouptable. The word line range included in each element is previouslyregistered in the element-group table. After that, the controller 200makes the element belong to the new group Grk in step S206.

In the flowchart of FIG. 46, if the determination in step S154 is YES(i.e., the element belongs to the group), the controller 200 determines,in step S158, whether the detection reservation flag of the element is“1” or not, by referring to the element-group table (FIG. 44A). If thedetermination in step S154 is YES (i.e., the detection reservation flagis “1”), the controller 200 sets the detection reservation flag to 0 instep S160. After that, the controller 200 executes second joiningprocess in step S162.

In the second joining process shown in FIG. 47B, the controller 200instructs, in step S302, the flash memory chip 100 to execute the“programming for parameter tuning” for the element. The write parametervalue obtained by the “programming for parameter tuning” is set into theregister 170A. The controller 200 determines, in step S304, whether ornot a group representative value matching the write parameter valueobtained by the “programming for parameter tuning” exists in the grouptable (FIG. 44B). If the determination in step S304 is YES, the flowends (i.e., the element stays). If the determination in step S304 is NO,the controller 200 evicts the element from the group (the element isremoved from the group) in step S306.

In step S308, the controller 200 determines whether or not a grouprepresentative value matching the write parameter value obtained by the“programming for parameter tuning” exists in the group table (FIG. 44B)(here, it is assumed to exist in a group Grk) and the element satisfiesthe group condition of the group Grk. If the determination in step S308is YES (i.e., the group representative value exists and the elementsatisfies the group condition), the controller 200 updates each table tomake the element belong to the group Grk in step S310. An example ofupdating the table will be explained later.

If the determination in step S308 is NO (i.e., the group representativevalue does not exist or the element does not satisfy the groupcondition), the controller 200 updates, in step S312, each table tocreate a new group Grk including the write parameter value obtained bythe “programming for parameter tuning” as the group representativevalue. The group condition of the new group Grk is set by referring tothe P/E cycle table, the P/E cycle interval table, and the element-grouptable. After that, the controller 200 makes the element belong to thenew group Grk in step S310. In step S310, the second joining process hasended.

In the flowchart of FIG. 46, if the determination in step S158 is NO(i.e., the detection reservation flag is “0”), the controller 200determines, in step S166, whether the element satisfies the groupcondition of the group to which the element belongs or not, by referringto the group table (FIG. 44B). If the determination result in step S166is YES (i.e., the element satisfies the group condition of the group towhich the element belongs), the controller 200 instructs, in step S178,the flash memory chip 100 to execute the “programming with tunedparameter” using the group representative value of the group to whichthe element belongs. In this case, the element stays in the group towhich the element belongs. After that, the controller 200 sets, in stepS182, the detection reservation flag of this element in theelement-group table to “1”. However, the controller 200 may set thedetection reservation flag of this element in the element-group table to“0” in step S182. Furthermore, the controller 200 randomly sets thedetection reservation flag of this element in the element-group table to“1” in step S182. If the detection reservation flag is set to “0”, thenumber of executions of the sampling inspection is reduced.

If the determination in step S166 is NO (i.e., the element does notsatisfy the group condition of the group to which the element belongs),the controller 200 determines, in step S168, whether the successiondestination of the group to which the element belongs is defined or not,by referring to the succession destination table (FIG. 44C). If thedetermination in step S168 is NO (i.e., the succession destination ofthe group to which the element belongs is not defined), the controller200 executes the first joining process in step S170. After the firstjoining process, the controller 200 updates, in step S172, thesuccession destination table to set the succession destination to agroup Grk (referring to step S206 in FIG. 47A) (succession destinationis held).

If the determination in step S168 is YES (i.e., the group Grk is definedas the succession destination), in step S176, the controller 200 updatesthe element-group table to change the group to which the element belongsto the group Grk (succession) and instructs the flash memory chip 100 toexecute the “programming with tuned parameter” using the grouprepresentative value of the group Grk.

FIG. 48A to FIG. 54D show examples of updating the tables in variousstates of moving elements between groups.

FIG. 48A to FIG. 48E show states of the element-group table, the grouptable, the succession destination table, the P/E cycle interval table,and the P/E cycle table when an element which does not belong to anygroup joins a newly created group, for example, in steps S204, S208, andS206 in the first joining process in step S156 shown in FIG. 46.

Groups #0 and #1 exist, entries of hatch lines (third rows) in theelement-group table, the group table, and the succession destinationtable do not exist, and the target element is the element including wordline #2 of block #0. Since the element including word line #2 of block#0 does not belong to any group as shown in FIG. 48A (NO in step S154 ofFIG. 46), “programming for parameter tuning” is executed for the elementincluding word line #2 of block #0 in step S202 of FIG. 47A and a writeparameter value V0 is detected. It is determined in step S204 of FIG.47A whether or not a group with a group representative value matchingthe write parameter value V0 is present and whether or not the elementincluding word line #2 of block #0 satisfies the group condition of thegroup with the group representative value matching the write parametervalue V0 if the group with the group representative value matching thewrite parameter value V0 is present. The group representative value ofthe groups #0 and #1 is V0 as shown in FIG. 48B, the element includingword line #2 of block #0 does not satisfy any of the group conditions(word line range) of the groups #0 and #1, a new group #2 with the grouprepresentative value matching the write parameter value V0 is createdand the element made to _(belong) to the new group #2. As a result, anentry of group 2 is added to the group table as shown in FIG. 48B and anentry of group 2 is added to the succession destination table as shownin FIG. 48C.

The word line range and the P/E cycle range as the group condition ofthe group #2 are set with reference to the P/E cycle table (entry ofblock #0 in FIG. 48E) and the P/E cycle interval table (entry of wordline range 2-31 in FIG. 48D). If the number of P/E cycles is smallerthan the P/E cycle interval, the P/E cycle range as the group conditionis obtained from the P/E cycle interval table. On the other hand, if thenumber of P/E cycles is 1020, the P/E cycle range as the group conditionis calculated from the P/E cycle interval table and the number of P/Ecycles. For example, when M (integer) satisfyingN×M≤k<N×(M+1) isobtained, the P/E cycle interval is set to a range from N×M to N×(M+1).

FIG. 49A to FIG. 49C show states of the element-group table, the grouptable, and the P/E cycle table when an element which does not belong toa group joins an existing group, for example, in steps S204 and S206 inthe first joining process in FIG. 47A in step S156 shown in FIG. 46. Itis assumed that entries of hatch lines (third row) in the element-grouptable do not exist (not defined), and the target element is the elementincluding word line #0 of block #1.

Since the element including word line #0 of block #1 does not belong toany group (NO in step S154 shown in FIG. 46), the “programming forparameter tuning” is executed and program voltage V0 is detected in stepS202 shown in FIG. 47A. The controller 200 determines whether the grouprepresentative value of the group is V0 or not. If the grouprepresentative value matches the write parameter value V0, thecontroller 200 determines whether the element satisfies the groupcondition or not, in step S204 shown in FIG. 47A. The grouprepresentative value of each of groups #0 and #1 is VO. The elementincluding word line #0 satisfies the sub-condition (word line #0) ofgroup #0, the number of P/E cycles of the element (block #1) is 20, andsatisfies the sub-condition (P/E cycle range: 0 to 100) of group #0. Tomake the element belong to group #0, an entry of group #0 of the wordline #0 represented by hatch lines is added to the element-group tablein step S206 shown in FIG. 47A.

FIG. 50A to FIG. 50E show states of the element-group table, the grouptable, the succession destination table, the P/E cycle interval table,and the P/E cycle table wherein an element belonging to a group isreleased from the group and joins a newly created group. For example,FIG. 50A to FIG. 50E show states of the tables wherein the determinationin step S154 is YES, the determination in step S158 is NO, thedetermination in step S166 is NO, and the determination in step S168 isNO, in the flowchart of FIG. 46. Groups #0 and #1 exist (defined),entries of hatch lines (third row) in the group table and the successiondestination table do not exist (not defined), and the target element isthe element including word line #0 of block #1.

The element (including word line #0 of block #1) belongs to group #0,but the number of P/E cycles of the element (block #1) is 101 and theelement does not satisfy the sub-condition (P/E cycle range: 0 to 100)of group #0 anymore. Since the succession destination of group #0 is notdefined, the “programming for parameter tuning” is executed for theelement and a write parameter value V1 is detected. A new group #2wherein the group representative value matches V1 is created, and theelement is made to belong to group #2. As a result, the group indexcorresponding to word line #0 of block #1 is changed from #0 to #2 inthe element-group table, as shown in FIG. 50A, and an entry of group #2represented by hatch lines is added to the group table, as shown in FIG.50B. A group condition of group #2 including the sub-condition of wordline range and the sub-condition of P/E cycle range is set by referringto the P/E cycle table (entry of block #1 represented by hatch lines),as shown in FIG. 50E and the P/E cycle interval table (word line #0represented by hatch lines), as shown in FIG. 50D. The successiondestination group of group #0 in the succession destination table is setto group #2, as shown in

FIG. 50C.

FIG. 51A and FIG. 51B show states of the element-group table and thegroup table when an element stays in an original group to which theelement belongs as a result of the sampling inspection, for example,wherein the determination in step S304 in the second joining process inFIG. 47B in step S162 shown in FIG. 46 is NO. The target of the samplinginspection is assumed to be an element including word line #0 of block#0.

Since the element (including word line #0 of block #0) belongs to group#0 and the detection reservation flag is “1”, the detection reservationflag is set to “0” in step S160, as shown in FIG. 51A. The “programmingfor parameter tuning” is executed for the element and a write parametervalue V0 is detected in step S302 in FIG. 47B. Since the write parametervalue V0 matches the group representative value V0 of group #0, theelement stays in group #0.

FIG. 52A to FIG. 52D show states of the element-group table, the grouptable, the P/E cycle table, and the P/E cycle interval table when anelement is evicted from a group to which the element currently belongsand joins a newly created group as a result of sampling inspection. Forexample, FIG. 52A to FIG. 52D show states of the tables in step S310 inthe second joining process in step S162 shown in FIG. 46. In the secondjoining process shown in FIG. 47B, it is assumed that the determinationin step S304 is NO and the determination in step S308 is YES. The targetof the sampling inspection is assumed to be the element including wordline #0 of block #0 in the state wherein group #0 exists (defined).

Since an element (including word line #0 of block #0) belongs to group#0 and the detection reservation flag is “1”, the detection reservationflag is set to “0” in step S160, as shown in FIG. 52A. The “programmingfor parameter tuning” is executed for the element and a write parametervalue V1 is detected in step S302 shown in FIG. 47B. Since the writeparameter value V1 does no match the group representative value V0 ofgroup #0, the element is evicted from group #0. If a group with a grouprepresentative value that matches V1 is not found, a new group #1 iscreated. As a result, as shown in FIG. 52B, an entry of group #1represented by hatch lines is added to the group table, and as shown inFIG. 52A, the belonging group of the element including word line range#0 of block #0 is changed from group #0 to group #1, by changing itsgroup index from #0 to #1 in the element-group table. A group conditionof group #1 including the sub-condition of word line range and thesub-condition of P/E cycle range is set by referring to the P/E cycletable (entry of block #0) shown in FIG. 52C and the P/E cycle intervaltable (word line range and P/E cycle interval) shown in FIG. 52D.

FIG. 53A to FIG. 53D show states of the element-group table, the grouptable, the succession destination table, and the P/E cycle table whereinan element which belongs to a group having the succession destinationundefined does not satisfy the group condition and is made to change thebelonging group to another group, for example, in steps S170 and S172 inFIG. 46. The target element is assumed to be an element including wordline #0 of block 0.

The element (including word line #0 of block #0) belongs to group #0,but the number of P/E cycles of block #0 (101) is outside the P/E cyclerange (0 to 100) of group #0, the element is evicted from group #0.Since the succession destination of group #0 is not defined, the“programming for parameter tuning” is executed for the element and awrite parameter value V1 is detected. It is determined whether a grouphaving the group representative value matching V1 exists in the grouptable or not. If such a group exists, it is determined whether theelement satisfies the group condition of the group or not. The grouprepresentative value of group #2 is V1. The number of P/E cycles of theelement including block #0 is 101 and satisfies the sub-condition (P/Ecycle range: 101 to 200) of group #2, and the word line #0 of theelement also satisfies the sub-condition (word line #0) of group #2. Asa result, to make the element belong to group #2, the group index of theelement including word line #0 of block #0 is changed from group #0 togroup #2 in the element-group table shown in FIG. 53A, and thesuccession destination of group #0 is set to group #2 in the successiondestination table shown in FIG. 53C.

If a group wherein a group representative value matches V1 does notexist, a new group wherein the group representative value is V1 iscreated.

FIG. 54A to FIG. 54D show states of the element-group table, the grouptable, the succession destination table, and the P/E cycle table whereinan element which belongs to a group having the succession destinationdefined does not satisfy the group condition and succession isperformed, for example, in steps S168 and S176 in FIG. 46. The targetelement is assumed to be the element including word line #0 of block #0.

The element (including word line #0 of block #0) belongs to group #0,but the number of P/E cycles of block #0 (101) is outside thesub-condition of P/E cycle range (0 to 100) of group #0, the element isreleased from group #0. Since group #2 is defined as the successiondestination of group #0, the group index of the element including wordline #0 of block #0 is changed from group #0 to group #2 in theelement-group table shown in FIG. 54A. In the succession, the“programming for parameter tuning” is not executed but the group towhich the element belongs is changed. The interval of executing the“programming for parameter tuning” can be thereby shortened.

As explained above, the “programming for parameter tuning” is executedfor the element which does not belong to a group, and the writeparameter value is obtained. When the write parameter value is obtained,it is determined to which group the element is made to belong based onthe obtained write parameter value. If there is no existing group towhich the element is made to belong, a new group is created and theelement is made to belong to the newly created group.

A group condition is defined for a group and it is assumed thatcharacteristic of a memory cell transistor of an element of the grouphas changed when the element does not satisfy the group condition thatthe characteristic has not changed while the element satisfies the groupcondition. Before writing data to the element, it is determined whetherthe element satisfies the group condition of the group or not. If theelement does not satisfy the group condition, the element is made tochange the belonging group to another group. For this reason, thecontroller 200 can dynamically change elements included in a group as torecompose groups, based on the result of the “programming for parametertuning” of the element and the group condition and, even when thecharacteristic of the element including the group has changed, thecontroller 200 can reduce an average program time by executing the“programming with tuned parameter” for other elements in the group byusing the write parameter value obtained by the “programming forparameter tuning” executed for an element in the group.

It is assumed in the above description with reference to FIG. 24 toFIGS. 54A to 54D that data of plural bits are collectively written intoa memory cell with a single program command. Data of plural bits may bewritten into a memory cell in multiple steps. It is assumed that the“programming for parameter tuning” is executed at the first step ofwriting in multiple steps. A threshold voltage distribution obtained atthe first step of writing differs from a threshold voltage distributionobtained at another step. Thus, the initial value of the program voltage“Initial Vpgm” relating to the appropriate write parameter at the firststep of writing differs from the initial value of the program voltagerelating to the appropriate write parameter value at said another step.The group recomposition shown in FIG. 46 is executed at the first stepof writing. At said another step of writing, the “programming with tunedparameter” may be executed. For the “programming with tuned parameter,”the appropriate write parameter value at said another step may beestimated based on the write parameter value obtained by the“programming for parameter tuning” executed at the first step.Alternatively, at each step of multiple steps of writing, the “write forparameter tuning” may be executed, and individual group tables may becreated for each step. Groups are created for each state and thus it ispossible to determine whether the “programming for parameter tuning” orthe “programming with tuned parameter” should be executed at each stepof the multiple steps of writing.

The present invention is not limited to the embodiments described abovebut the constituent elements of the invention can be modified in variousmanners without departing from the spirit and scope of the invention.Various aspects of the invention can also be extracted from anyappropriate combination of a plurality of constituent elements disclosedin the embodiments. Some constituent elements may be deleted in all ofthe constituent elements disclosed in the embodiments. The constituentelements described in different embodiments may be combined in any way.

What is claimed is:
 1. A memory system comprising: a nonvolatilesemiconductor memory; and a controller configured to control thesemiconductor memory, wherein the semiconductor memory comprises: amemory cell; and a write circuit configured to write data to the memorycell by applying a program voltage to the memory cell and comparing athreshold voltage of the memory cell with a first reference voltagecorresponding to the write data, wherein the write circuit is configuredto execute a first programming operation to obtain a value of a writeparameter by comparing the threshold voltage of the memory cell with asecond reference voltage, the second reference voltage being differentfrom the first reference voltage.
 2. The memory system of claim 1,wherein a level of the program voltage is varied during one execution ofthe first programming operation.
 3. The memory system of claim 1,wherein the controller comprises a first memory; and the controller isfurther configured to obtain the value of the write parameter from thesemiconductor memory and store the obtained value of the write parameterinto the first memory.
 4. The memory system of claim 1, wherein thesemiconductor memory comprises memory cells; and the write parametercomprises a level of the program voltage at a time during the firstprogramming operation and when a highest voltage of threshold voltagedistribution of memory cells exceeds the second reference voltage. 5.The memory system of claim 1, wherein the write circuit is furtherconfigured to execute a second programming operation of making thethreshold voltage exceed the first reference voltage while varying alevel of the program voltage according to the write parameter during oneexecution of the first programming operation; and the controller isconfigured to supply, to the write circuit, a first instruction for thefirst programming operation or a second instruction for the secondprogramming operation.
 6. The memory system of claim 5, wherein thefirst instruction comprises a first command sequence including a firstcommand for the first programming operation; the second instructioncomprises a second command sequence including a second command for thesecond programming operation, the second command being different fromthe first command.
 7. The memory system of claim 5, wherein one of thefirst instruction and the second instruction comprises a first commandsequence including a prefix command; and the other of the firstinstruction and the second instruction comprises a second commandsequence not including the prefix command.
 8. The memory system of claim5, wherein the semiconductor memory comprises a plurality of elements,the plurality of elements being divided into groups; the controllercomprises a first memory; and the controller is further configured tostore, into the first memory, values of the write parameter of each ofthe groups, and supply, to the write circuit, the second instruction towrite data into a write target group among the groups and a value of thewrite parameter of the write target group, the value of the writeparameter of the write target group being stored in the first memory. 9.The memory system of claim 8, wherein the controller is furtherconfigured to supply, to the write circuit, the first instruction towrite data into the write target group when the value of the writeparameter of the write target group is not stored in the first memory.10. The memory system of claim 8, wherein the controller is furtherconfigured to create a new group of elements when a write target elementdoes not satisfy group condition of any of the groups, the new grouphaving a group condition satisfied by the write target element.
 11. Thememory system of claim 8, wherein the controller is further configuredto supply, to the write circuit, the first instruction to write datainto a first element in a first group among the groups, obtain a valueof the write parameter of the first element; and make the first elementstay in the first group or join a second group according to a comparisonresult of the value of the write parameter of the first element obtainedby the first programming operation and the values of the write parameterstored in the first memory.
 12. The memory system of claim 8, whereinthe controller is further configured to supply, to the write circuit,the first instruction to write data into a first element in a firstgroup among the groups when the first element does not satisfy a groupcondition of the first group.
 13. The memory system of claim 5, whereinthe semiconductor memory comprises a plurality of memory cells, theplurality of memory cells being divided into one or more string units;the string units are commonly connected to word lines; the word linesare divided into groups, a first group among the groups comprising oneor more word lines including a first word line; and the controller isfurther configured to supply, to the write circuit, the firstinstruction to write data into first write target cells among memorycells in the first group and the second instruction to write data intosecond write target cells and third write target cells, both the secondwrite target cells and the third write target cells being among memorycells in the first group, the first write target cells being connectedto the first word line and being included in a first string unit amongthe string units, the second write target cells being connected to thefirst word line and being included in a string unit other than the firststring unit, the third write target cells being connected to a word lineother than the first word line.
 14. The memory system of claim 5,wherein the semiconductor memory comprises a plurality of memory cells,the plurality of memory cells being divided into groups; and thecontroller is further configured to execute a program-and-erase cyclefor the plurality of memory cells, and supply, to the write circuit, thefirst instruction to write data into memory cells in a group among thegroups once in a plurality of executions of the program-and-erase cyclesin the group.
 15. The memory system of claim 5, wherein thesemiconductor memory comprises a plurality of memory cells, theplurality of memory cells being divided into groups; the controller isfurther configured to execute a program-and-erase cycle for theplurality of memory cells, supply, to the write circuit, the firstinstruction to write data into memory cells in a first group among thegroups once at a first sequence number of execution among a first numberof executions of the program-and-erase cycle in the first group, andsupply, to the write circuit, the first instruction to write data intomemory cells second group among the groups once at a second sequencenumber of execution among the first number of executions of theprogram-and-erase cycle in the second group, the first sequence numberbeing different from the second sequence number.
 16. The memory systemof claim 15, wherein the plurality of memory cells are connected to wordlines; the groups include a third group, the third group comprising oneor more word lines including a first word line; and the controller isfurther configured to supply, to the write circuit, the firstinstruction to write data into memory cells once in a plurality ofexecutions of the program-and-erase cycle in the third group, the memorycells being in the third group and being connected to the first wordline.
 17. The memory system of claim 5, wherein the semiconductor memorycomprises a plurality of memory cells, the plurality of memory cellsbeing connected to word lines, the word lines divided into groups, thegroups including a first group and a second group; the controller isfurther configured to execute a program-and-erase cycle, supply, to thewrite circuit, the first instruction to write data into memory cells inthe first group once in a first number of executions of theprogram-and-erase cycle in the first group, and supply, to the writecircuit, the first instruction to write data into memory cells in thesecond group once in a second number of executions of theprogram-and-erase cycle in the second group; and the first number ofexecutions is different from the second number of executions isdifferent from the second number of executions.
 18. The memory system ofclaim 17, wherein the word lines include a first word line and a lastword line, the word lines are arranged in an order from the first wordline to the last word line, and, in a case where word lines of the firstgroup are closer to the first word line than word lines of the secondgroup, the first number of executions is smaller than the second numberof executions.
 19. The memory system of claim 17, wherein the word linesare divided into a first tier of word lines and a second tier of wordlines, each of the first tier of word lines and the second tier of wordlines includes a first word line and a last word line, word lines ineach of the first tier and the second tier are arranged in an order fromthe first word line to the last word line, in a case where word lines ofthe first group and word lines of the second group belong to the firsttier, and a word line of the first group is closer to the first wordline of the first tier than the word lines of the second group, thefirst number of executions is smaller than the second number ofexecutions, and, in a case where the word lines of the first group andthe word lines of the second group belong to the second tier, and a wordline of the first group is closer to the first word line of the secondtier than the word lines of the second group, the first number ofexecutions is smaller than the second number of executions.